Clean up wording of symbol descriptions (fixes nzakas#135)

Author: nzakas

manuscript/04-Symbols.md

@@ -84,7 +84,7 @@ With computed property names in object literals, symbols are very easy to work w
 
 You may find that you want different parts of your code to use the same symbols. For example, suppose you have two different object types in your application that should use the same symbol property to represent a unique identifier. Keeping track of symbols across files or large codebases can be difficult and error-prone. That's why ECMAScript 6 provides a global symbol registry that you can access at any point in time.
 
-When you want to create a symbol to be shared, use the `Symbol.for()` method instead of calling `Symbol()`. The `Symbol.for()` method accepts a single parameter, which is a string identifier for the symbol you want to create (this value doubles as the description). For example:
+When you want to create a symbol to be shared, use the `Symbol.for()` method instead of calling `Symbol()`. The `Symbol.for()` method accepts a single parameter, which is a string identifier for the symbol you want to create (this value is also used as the description). For example:
 
 ```js
 var uid = Symbol.for("uid");