Clearer rule severity for React Compiler eslint plugin (#6870)

Using "off", "warn", "error" has been supported for many years now - I think this helps improve the readability :) 

I completely forgot what rule: 2 meant - had to find it at 

https://eslint.org/docs/latest/use/configure/rules

src/content/learn/react-compiler.md

@@ -121,7 +121,7 @@ module.exports = {
     'eslint-plugin-react-compiler',
   ],
   rules: {
-    'react-compiler/react-compiler': 2,
+    'react-compiler/react-compiler': "error",
   },
 }
 ```