Fix default_branch config value name in README (#29)

Author: Jackenmen

readme.rst

@@ -134,7 +134,7 @@ Available config options::
                    on pull-request xxxx.
                    For projects using GitHub Issues, this option can be disabled.
 
-   repo            Project's default branch name,
+   default_branch  Project's default branch name,
                    e.g "devel" for https://github.com/ansible/ansible
                    ("main" by default)