Update README.md

Update README.md to standardize the use of single versus double quotes.

P.S. Sorry if this is a very trivial PR.

README.md

@@ -16,7 +16,7 @@ var extend = require("xtend")
 // extend returns a new object. Does not mutate arguments
 var combination = extend({
     a: "a",
-    b: 'c'
+    b: "c"
 }, {
     b: "b"
 })