Fix trivial grammar error.

make-your-own-gem.md

@@ -218,7 +218,7 @@ includes the `lib` directory for us, so end users don't need to worry about
 configuring their load paths. However, if we're running our code outside of 
 RubyGems, we have to configure things ourselves. It's possible to manipulate 
 the `$LOAD_PATH` from within the code itself, but that's considered an 
-anti-pattern in most cases. There's many more anti-patterns (and good patterns!) 
+anti-pattern in most cases. There are many more anti-patterns (and good patterns!) 
 for gems, explained in [this guide](/patterns).
 
 If you've added more files to your gem, make sure to remember to add them to