chore(ci): update docs on push to master

The frequency of updating the docs was previously reduced to avoid noise
in the Pact Python Slack channel; however, it means that blog posts, or
minor corrections to any docstring do not get updated.

I am reverting the previous change and will investigate the possibility
of publishing updates to Slack, instead of Slack subscribing to all
deployments.

Signed-off-by: JP-Ellis <josh@jpellis.me>

.github/workflows/docs.yml

@@ -48,7 +48,7 @@ jobs:
   publish:
     name: Publish docs
 
-    if: github.event_name == 'push' && startsWith(github.event.ref, 'refs/tags/v')
+    if: github.ref == 'refs/heads/master'
 
     needs: build
     runs-on: ubuntu-latest