docs(CONTRIBUTING.md) : fix grammatical error in refactor description

Grammar:
- Consistency using "neither" with "nor"

Author: dvandyke

CONTRIBUTING.md

@@ -180,7 +180,7 @@ Must be one of the following:
 * **docs**: Documentation only changes
 * **style**: Changes that do not affect the meaning of the code (white-space, formatting, missing
   semi-colons, etc)
-* **refactor**: A code change that neither fixes a bug or adds a feature
+* **refactor**: A code change that neither fixes a bug nor adds a feature
 * **perf**: A code change that improves performance
 * **test**: Adding missing tests
 * **chore**: Changes to the build process or auxiliary tools and libraries such as documentation