image-publish.yml: Update cvmfs-removal example

Author: ric-evans

.github/workflows/image-publish.yml

@@ -43,7 +43,7 @@ on:
             The image **TAGS** (i.e. 'bar' in 'baz/foo:bar') to remove on CVMFS -- newline-delimited
             tags can be a pattern-based or verbatim
           SEE: https://github.com/WIPACrepo/build-singularity-cvmfs-action
-          EXAMPLE: '$ {{ github.ref_name }}-[SHA]' (no space after $)
+          EXAMPLE: '$ {{ github.event.ref }}-[SHA]' (no space after $)
         type: string
         required: false
       free_disk_space: