Add missing "the" and fix comma

Author: sjsyrek

content/docs/optimizing-performance.md

@@ -66,7 +66,7 @@ Then, to create a production build, add the `-p` flag to the `build` command:
 brunch build -p
 ```
 
-Remember that you only need to do this for production builds. You shouldn't pass `-p` flag or apply this plugin in development because it will hide useful React warnings, and make the builds much slower.
+Remember that you only need to do this for production builds. You shouldn't pass the `-p` flag or apply this plugin in development, because it will hide useful React warnings and make the builds much slower.
 
 ### Browserify