Make the instructions to regenerate feature flags easier to see in the docs (facebook#45059)

Summary:
Pull Request resolved: facebook#45059

Changelog: [internal]

By moving the command to a code block it's going to be easier to see it when quickly reading the README.

Reviewed By: cortinico

Differential Revision: D58779883

fbshipit-source-id: e912a58641245c6d7dc158f7af0a722e438a0cc3

Author: rubennorte

packages/react-native/scripts/featureflags/README.md

@@ -32,9 +32,13 @@ module.exports = {
 };
 ```
 
-After any changes to this definitions, the code that provides access to them
-must be regenerated running `yarn featureflags-update` from the `react-native`
-repository.
+**After any change to these definitions**, the code that provides access to them
+must be regenerated running this from the `react-native`
+repository:
+
+```shell
+yarn featureflags-update
+```
 
 ## Access