Suppress a RuboCop offense

This commit suppresses the following RuboCop offense:

```console
$ bundle exec rake
(snip)

Offenses:

spec/rubocop/ast/procarg0_node_spec.rb:4:50: C: [Correctable] InternalAffairs/NodeFirstOrLastArgument:
Use #first_argument instead of #arguments.first.
  let(:procarg0_node) { parse_source(source).ast.arguments.first }
                                                 ^^^^^^^^^^^^^^^

163 files inspected, 1 offense detected, 1 offense autocorrectable
```

spec/rubocop/ast/procarg0_node_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 RSpec.describe RuboCop::AST::Procarg0Node, :ruby27 do
-  let(:procarg0_node) { parse_source(source).ast.arguments.first }
+  let(:procarg0_node) { parse_source(source).ast.first_argument }
 
   describe '.new' do
     context 'with a block' do