Fix a minor English grammar problem in user-facing text

[cwcowell]

Correcting wording of user-facing text when editing an ignored file.

For details on this tricky English grammar rule about restrictive vs. nonrestrictive clauses, see https://www.grammarly.com/blog/which-vs-that/

Fixes this github issue: https://github.com/JetBrains/idea-gitignore/issues/868