Merge pull request getify#132 from rcorp/typoCh6ThisAndObjectEssense

Typo:essense

this & object prototypes/ch6.md

@@ -11,7 +11,7 @@ As a brief review of our conclusions from Chapter 5, the `[[Prototype]]` mechani
 
 This linkage is exercised when a property/method reference is made against the first object, and no such property/method exists. In that case, the `[[Prototype]]` linkage tells the engine to look for the property/method on the linked-to object. In turn, if that object cannot fulfill the look-up, its `[[Prototype]]` is followed, and so on. This series of links between objects forms what is called the "prototype chain".
 
-In other words, the actual mechanism, the essense of what's important to the functionality we can leverage in JavaScript, is **all about objects being linked to other objects.**
+In other words, the actual mechanism, the essence of what's important to the functionality we can leverage in JavaScript, is **all about objects being linked to other objects.**
 
 That single observation is fundamental and critical to understanding the motivations and approaches for the rest of this chapter!