Clarify sentence in CONTRIBUTING (github-linguist#6887)

docs: clarify sentence

CONTRIBUTING.md

@@ -166,7 +166,7 @@ If you can, try to reproduce the highlighting problem in the text editor that th
 You can also try to fix the bug yourself and submit a pull-request.
 [TextMate's documentation](https://manual.macromates.com/en/language_grammars) offers a good introduction on how to work with TextMate-compatible grammars.
 Note that Linguist uses [PCRE](https://www.pcre.org/) regular expressions, while TextMate uses [Oniguruma](https://github.com/kkos/oniguruma).
-Although they are mostly compatible there might be some differences in syntax and semantics between the two.
+Although they are mostly compatible, there may be occasional differences in syntax and semantics between the two.
 Linguist's grammar compiler will highlight any problems when the grammar is updated.
 
 Once the bug has been fixed upstream, we'll pick it up for GitHub in the next release of Linguist.