Fix typo/wording in README.md (#367)

This was recently changed from "massaging giant hash structures" to "messaging
giant hash structures" in 955acd4 / #366. "Massaging," while correct, only makes
sense in light of a somewhat less common scondary definition meaning "to
manipulate". "Messaging" doesn't really make sense at all. Hopefully
"manipulating" (as introduced here) is the best of all.

Author: davidrunger2

README.md

@@ -1,7 +1,7 @@
 # Jbuilder
 
 Jbuilder gives you a simple DSL for declaring JSON structures that beats
-messaging giant hash structures. This is particularly helpful when the
+manipulating giant hash structures. This is particularly helpful when the
 generation process is fraught with conditionals and loops. Here's a simple
 example: