chore(ci): update workflow name (#13430)

Update name to reflect recent change to post submit check. Followup to
#13422.

Author: zhumin8

.github/workflows/librarian_generation_check.yaml

@@ -11,7 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-name: Librarian - Generate diff check on pull requests
+name: Librarian - Generate diff check on main
 on:
   push:
     branches: