Update 'Arrow Function' wording for clarity. [ci skip]

(minor doc change).  Saw a surprising number of issues and stack overflow questions about using arrow functions, so updated the documentation to be explicit about the lexical binding of 'this'.

Author: merriam

docs/index.md

@@ -254,7 +254,7 @@ describe('Array', function() {
 
 ## Arrow Functions
 
-Passing [arrow functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) ("lambdas") to Mocha is discouraged.  Due to the lexical binding of `this`, such functions are unable to access the Mocha context.  For example, the following code will fail due to the nature of lambdas:
+Passing [arrow functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) ("lambdas") to Mocha is discouraged.  Lambdas lexically bind `this` and cannot access the Mocha context.  For example, the following code will fail:
 
 ```js
 describe('my suite', () => {