Update PR template for new branching strategy.

.github/pull_request_template.md

@@ -2,15 +2,14 @@
 Thank you for contributing to the OpenAPI Specification!
 
 Please make certain you are submitting your PR on the correct
-branch and file:
+branch, to the files under the "src/" directory (which is not
+present on the main branch, only on the development branches).
 
-* 3.0.x spec: v3.0.4-dev branch, versions/3.0.4.md
-* 3.1.x spec: v3.1.1-dev branch, versions/3.1.1.md
-* 3.2.0 spec: v3.2.0-dev branch, versions/3.2.0.md
-* 3.0 schema: main branch, schemas/v3.0/...
-* 3.1 schema: main branch, schemas/v3.1/...
+* 3.1.x spec and schemas: v3.1-dev branch
+* 3.2.x spec and schemas: v3.2-dev branch
 * registry templates: gh-pages branch, registry/...
 * registry contents: gh-pages branch, registries/...
+* process documentation and build infrastructure: main
 
 Note that we do not accept changes to published specifications.
 -->