Allow alternative spelling for `pull request`

[mkokryashkin]

Checkpatch outputs the following for the GraphQL query in tarantool/tarantool#8514:

ERROR:TYPO_SPELLING: 'PullRequest' may be misspelled - perhaps 'Pull request'?
#166: FILE: tools/stale_pr_date_setter.py:111:
+                        ...on PullRequest
                              ^^^^^^^^^^^

That's a false positive error, PullRequest spelling should be allowed.