Contributing: update vale error list

Signed-off-by: Lynette Miles <[email protected]>

Author: esmerel

CONTRIBUTING.md

@@ -139,6 +139,7 @@ Most Vale tests are at the `suggestion` level and won't block merging.
 The following tests are at a `error` level and will prevent merging:
 
 - [AmSpelling](https://developers.google.com/style/word-list)
+- [Ampersand](https://developers.google.com/style/word-list#ampersand)
 - [Emdash](https://github.com/errata-ai/Google/blob/master/Google/EmDash.yml)
 - [Endash](https://github.com/errata-ai/Google/blob/master/Google/EmDash.yml)
 - [Exclamation](https://github.com/errata-ai/Google/blob/master/Google/Exclamation.yml)