Update the guide to mention the showDiff argument of assert

data/guide/plugins/assert.md

@@ -32,13 +32,15 @@ the assertion failed and the first error message will be thrown as part of a `ch
 Conversely, if the language chain was negated, it will consider `false` a pass and `true` a failure. 
 The second error message will be included in the thrown error instead.
 
-In all, the `assert` method accepts five arguments:
+In all, the `assert` method accepts six arguments:
 
 1. a boolean (result of a truth test)
 2. a string error message to be used if the first argument is `false`
 3. a string error message to be used if the assertion is negated and the first argument is `true`
 4. (optional) the expected value
 5. (optional) the actual value, which will default to `_obj`
+6. (optional) a boolean which indicates whether to display a diff in addition to the message if the first argument is `false`
+
 
 ### Composing Error Messages