chore: fix the tag format in librarian.yaml (#16687)

This was created by a bug in the migration tool.

Note that this won't have any impact until we're using librarian for
releasing, but we should get it fixed ahead of time.

Fixes googleapis/librarian#5341

Author: jskeet

librarian.yaml

@@ -24,7 +24,7 @@ release:
     - docs/README.rst
 default:
   output: packages
-  tag_format: '{name}: v{version}'
+  tag_format: '{name}-v{version}'
   python:
     common_gapic_paths:
       - samples/generated_samples