Make custom matchers heading levels consistent

All other docs use level 1 and level 2 headings.

Author: randycoulman

docs/8-custom-matchers.md

@@ -1,4 +1,4 @@
-### Custom argument matchers
+# Custom argument matchers
 
 In addition to the built-in argument matchers described along with
 [stubbing](5-stubbing-results.md#loosening-stubbings-with-argument-matchers),
@@ -17,7 +17,7 @@ actual invocation if it passes lodash's deep
 
 The examples in this document assume you've aliased `testdouble` to `td`.
 
-#### Example
+## Example
 
 Here's a naive implementation of a matcher named `isA` which will check whether
 the expected type of an argument matches the type of the argument actually passed
@@ -44,7 +44,7 @@ datePicker(new Date()) // 'good'
 datePicker(5) // undefined
 ```
 
-#### td.matchers.create API
+## td.matchers.create API
 
 The `create` function takes a configuration object with the following properties