filter-repo: consistent warning/error casing

Changed a single log that used all-caps

Signed-off-by: Diego Mateos <[email protected]>

Author: dimateos

git-filter-repo

@@ -4455,7 +4455,7 @@ class RepoFilter(object):
       ret = subproc.call([arg if arg != '""' else '' for arg in cmd.split()],
                          cwd=source_working_dir)
       if ret != 0: # pragma: no cover
-        m = _("WARNING: Fetching all refs from origin failed")
+        m = _("Warning: Fetching all refs from origin failed")
         print(m)
     if self._args.sensitive_data_removal:
       return