Update README.md for clang-format 20 (#14533)

README.md

@@ -153,7 +153,7 @@ GitHub Actions will verify that any code changes are done in a style-compliant
 way. Install `clang-format` and `mint`:
 
 ```console
-brew install clang-format@19
+brew install clang-format@20
 brew install mint
 ```