Fix typo in developer docs GitHub workflow (#15336)

Signed-off-by: Sean Quah <seanq@matrix.org>

.github/workflows/docs.yaml

@@ -80,7 +80,7 @@ jobs:
     needs:
       - pre
     steps:
-      - uses: action/checkout@v3
+      - uses: actions/checkout@v3
 
       - name: "Set up Sphinx"
         uses: matrix-org/setup-python-poetry@v1

changelog.d/15336.misc

@@ -0,0 +1 @@
+Add developer documentation for the Federation Sender and add a documentation mechanism using Sphinx.