Merge pull request docker#6954 from alecthegeek/patch-1

Fix broken link

Author: Tammy Fox

compose/reference/build.md

@@ -22,7 +22,7 @@ Services are built once and then tagged, by default as `project_service`. For
 example, `composetest_db`. If the Compose file specifies an
 [image](/compose/compose-file/index.md#image) name, the image is
 tagged with that name, substituting any variables beforehand. See [variable
-substitution](#variable-substitution).
+substitution](/compose/compose-file/#variable-substitution).
 
 If you change a service's Dockerfile or the contents of its
 build directory, run `docker-compose build` to rebuild it.