docs(CONTRIBUTING): correct spelling and grammar

Summary:
  - Use properly capitalized GitHub brand name
  - Correctly negate two clauses using "nor" (caitp feels this may confuse
non-english speakers and need to be revised, but hopefully not)
  - Correctly end sentence with period

Closes #12497

Author: stevemao

CONTRIBUTING.md

@@ -202,7 +202,7 @@ format that includes a **type**, a **scope** and a **subject**:
 The **header** is mandatory and the **scope** of the header is optional.
 
 Any line of the commit message cannot be longer 100 characters! This allows the message to be easier
-to read on github as well as in various git tools.
+to read on GitHub as well as in various git tools.
 
 ### Revert
 If the commit reverts a previous commit, it should begin with `revert: `, followed by the header of the reverted commit. In the body it should say: `This reverts commit <hash>.`, where the hash is the SHA of the commit being reverted.
@@ -215,7 +215,7 @@ 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
@@ -233,7 +233,7 @@ The subject contains succinct description of the change:
 * no dot (.) at the end
 
 ### Body
-Just as in the **subject**, use the imperative, present tense: "change" not "changed" nor "changes"
+Just as in the **subject**, use the imperative, present tense: "change" not "changed" nor "changes".
 The body should include the motivation for the change and contrast this with previous behavior.
 
 ### Footer