package.json: Reformat to formatting used by npm

This makes it possible to do e.g. "npm install --save" without a lot of
whitespace noise in the resulting diff.

package.json

@@ -2,7 +2,10 @@
   "name": "pegjs",
   "version": "0.9.0",
   "description": "Parser generator for JavaScript",
-  "keywords":        ["parser generator", "PEG"],
+  "keywords": [
+    "parser generator",
+    "PEG"
+  ],
   "homepage": "http://pegjs.org/",
   "bugs": "https://github.com/pegjs/pegjs/issues",
   "license": "MIT",