docs: use assignment not comparison in deprecation strict mode example (#8599)

Author: gaishimo

docs/migrating-to-v22.md

@@ -22,7 +22,7 @@ This is useful to help you quickly locate any remaining usage of the deprecated
 Note that there may be modular API implementation errors within the react-native-firebase modules, this may still be useful as a troubleshooting aid when collaborating with the maintainers to correct these errors.
 
 ```js
-globalThis.RNFB_MODULAR_DEPRECATION_STRICT_MODE === true;
+globalThis.RNFB_MODULAR_DEPRECATION_STRICT_MODE = true;
 ```
 
 # Migrating to React Native modular API