Add quotations to the id of the gist helper

The test code for the gist helper surrounds the id with quotation marks
to treat the id as a string but the documentation and function
description do not. Implementing the example without quotation marks
results in <script src="https://gist.github.com/undefined.js"></script>
being returned.

README.md

@@ -737,7 +737,7 @@ Embed a GitHub Gist using only the id of the Gist
 **Example**
 
 ```handlebars
-{{gist 12345}}
+{{gist "12345"}}
 ```
 
 ### [{{jsfiddle}}](lib/code.js#L72)
@@ -2504,4 +2504,4 @@ Released under the [MIT license](https://github.com/assemble/handlebars-helpers/
 
 ***
 
-_This file was generated by [verb-generate-readme](https://github.com/verbose/verb-generate-readme), v0.1.30, on August 17, 2016._
+_This file was generated by [verb-generate-readme](https://github.com/verbose/verb-generate-readme), v0.1.30, on August 17, 2016._

lib/code.js

@@ -46,7 +46,7 @@ helpers.embed = function embed(fp, ext) {
  * Embed a GitHub Gist using only the id of the Gist
  *
  * ```handlebars
- * {{gist 12345}}
+ * {{gist "12345"}}
  * ```
  * @param  {String} `id`
  * @return {String}