Fix merge_request value in -30974-issue-search-by-number.yml

There was an syntax issue here which caused it can't be parsed properly.

cc !28302

yaml-lint(https://www.npmjs.com/package/yaml-lint):
```
✖ unknown tag !<!28302> at line 4, column 1:
    author: Riccardo Padovani
    ^
```

Online YAML Parser:(https://yaml-online-parser.appspot.com/)
```
ERROR:

could not determine a constructor for the tag '!28302'
  in "<unicode string>", line 3, column 16:
    merge_request: !28302                           ...
                   ^
```

changelogs/unreleased/-30974-issue-search-by-number.yml

 ---
 title: "Search issuables by iids"
-merge_request: !28302
+merge_request: 28302
 author: Riccardo Padovani
 type: fixed