Update docs workflow.

Build the docs based on the `master` branch instead of the `docs`
branch.

.github/workflows/build_docs.yaml

@@ -2,7 +2,7 @@ name: Build and deploy documentation
 on:
   push:
     branches:
-      - docs
+      - master
 jobs:
   build-docs:
     runs-on: ${{ matrix.os }}