fix: small typo in README (#4664)

This commit corrects a tiny typo in the capitalization of "GitHub".

Author: squat

README.md

@@ -1,6 +1,6 @@
 # The `<model-viewer>` project
 
-This is the main Github repository for the `<model-viewer>` web component and
+This is the main GitHub repository for the `<model-viewer>` web component and
 all of its related projects.
 
 **Getting started?** Check out the [`<model-viewer>`](packages/model-viewer) project!
@@ -124,4 +124,4 @@ Running `npm run dev` in `packages/model-viewer` on Windows throws error `ERROR:
 **Solution**
 (if you have one please make a PR!)
 
-</details>
+</details>