improve running test instructions and denote commands with $

CONTRIBUTING.rdoc

@@ -12,18 +12,18 @@ If you wish to run the unit and functional tests that come with Rake:
 * +cd+ into the top project directory of rake.
 * Install gem dependency using bundler:
 
-    bundle install  # Install bundler, minitest and rdoc
+    $ bundle install  # Install bundler, minitest and rdoc
 
-* Type one of the following:
+* Run the test suite
 
-    rake            # If you have run rake's tests
+    $ rake
 
 = Rubocop
 
 Rake uses Rubocop to enforce a consistent style on new changes being
 proposed. You can check your code with Rubocop using:
 
-  ./bin/rubocop
+  $ ./bin/rubocop
 
 = Issues and Bug Reports