docs: update references to main branch (KhronosGroup#5363)

The default branch was renamed from `master` to `main`.  Update some
stale references.

Signed-off-by: Sven van Haastregt <[email protected]>

CONTRIBUTING.md

@@ -3,7 +3,7 @@
 ## For users: Reporting bugs and requesting features
 
 We organize known future work in GitHub projects. See
-[Tracking SPIRV-Tools work with GitHub projects](https://github.com/KhronosGroup/SPIRV-Tools/blob/master/docs/projects.md)
+[Tracking SPIRV-Tools work with GitHub projects](https://github.com/KhronosGroup/SPIRV-Tools/blob/main/docs/projects.md)
 for more.
 
 To report a new bug or request a new feature, please file a GitHub issue. Please
@@ -46,7 +46,7 @@ sign the CLA until after you've submitted your code for review and a member has
 approved it, but you must do it before we can put your code into our codebase.
 
 See
-[README.md](https://github.com/KhronosGroup/SPIRV-Tools/blob/master/README.md)
+[README.md](https://github.com/KhronosGroup/SPIRV-Tools/blob/main/README.md)
 for instruction on how to get, build, and test the source. Once you have made
 your changes:
 
@@ -59,7 +59,7 @@ your changes:
 *   If your patch completely fixes bug 1234, the commit message should say
     `Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/1234` When you do
     this, the issue will be closed automatically when the commit goes into
-    master. Also, this helps us update the [CHANGES](CHANGES) file.
+    main. Also, this helps us update the [CHANGES](CHANGES) file.
 *   Watch the continuous builds to make sure they pass.
 *   Request a code review.
 
@@ -107,7 +107,7 @@ should pay particular attention to:
 
 ## For maintainers: Merging a PR
 
-We intend to maintain a linear history on the GitHub master branch, and the
+We intend to maintain a linear history on the GitHub main branch, and the
 build and its tests should pass at each commit in that history. A linear
 always-working history is easier to understand and to bisect in case we want to
 find which commit introduced a bug. The

docs/downloads.md

@@ -2,7 +2,7 @@
 
 ## Latest builds
 
-Download the latest builds of the [master](https://github.com/KhronosGroup/SPIRV-Tools/tree/master) branch.
+Download the latest builds of the [main](https://github.com/KhronosGroup/SPIRV-Tools/tree/main) branch.
 
 ### Release build
 | Windows | Linux | MacOS |

docs/projects.md

@@ -34,7 +34,7 @@ through the project workflow:
   ones.
 * They determine if the work for a card has been completed.
 * Normally they are the person (or persons) who can approve and merge a pull
-  request into the `master` branch.
+  request into the `main` branch.
 
 Our projects organize cards into the following columns:
 * `Ideas`: Work which could be done, captured either as Cards or Notes.
@@ -51,7 +51,7 @@ Our projects organize cards into the following columns:
   claimed by someone.
 * `Done`: Issues which have been resolved, by completing their work.
   * The changes have been applied to the repository, typically by being pushed
-  into the `master` branch.
+  into the `main` branch.
   * Other kinds of work could update repository settings, for example.
 * `Rejected ideas`: Work which has been considered, but which we don't want
   implemented.