[docs] Fix typo in CONTRIBUTING.md (mui#41670)

Author: adriancuadrado

CONTRIBUTING.md

@@ -25,7 +25,7 @@ Here are a few guidelines that will help you along the way.
 
 ## Code of conduct
 
-We has adopted the [Contributor Covenant](https://www.contributor-covenant.org/) as our code of conduct, and we expect project participants to adhere to it.
+We have adopted the [Contributor Covenant](https://www.contributor-covenant.org/) as our code of conduct, and we expect project participants to adhere to it.
 Please read [the full text](https://github.com/mui/.github/blob/master/CODE_OF_CONDUCT.md) to understand what actions will and will not be tolerated.
 
 ## A large spectrum of contributions