fix rubocop rule for rspec max nesting

This fixes the rule setting the limit for nested example groups in
RSpec. Its current setting is invalid and does not work.

test plan:
* verify Jenkins passes

Change-Id: If6e03df1d9fd66bc5aacf1f78e0918086c6c73de
Reviewed-on: https://gerrit.instructure.com/100780
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins
QA-Review: Jeremy Neander <jneander@instructure.com>
Product-Review: Jeremy Neander <jneander@instructure.com>

.rubocop.yml

@@ -175,7 +175,7 @@ RSpec/ExampleWording:
 RSpec/ExampleLength:
   Max: 16
 RSpec/NestedGroups:
-  Max: 5
+  MaxNesting: 5
 RSpec/DescribedClass:
   Enabled: false
 RSpec/DescribeClass: