fixed Undefined index: modname

This PR fixes a minor template error in the categories module,

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | 
| Refs tickets  | 
| License       | MIT
| Doc PR        | 

-Mark