docs, update "How to run Active Record tests". [ci skip]

activerecord/RUNNING_UNIT_TESTS.rdoc

@@ -16,7 +16,7 @@ To run a set of tests:
 You can also run tests that depend upon a specific database backend. For
 example:
 
-  $ bundle exec rake test_sqlite3
+  $ bundle exec rake test:sqlite3
 
 Simply executing <tt>bundle exec rake test</tt> is equivalent to the following:
 
@@ -24,6 +24,10 @@ Simply executing <tt>bundle exec rake test</tt> is equivalent to the following:
   $ bundle exec rake test:mysql2
   $ bundle exec rake test:postgresql
   $ bundle exec rake test:sqlite3
+
+Using the SQLite3 adapter with an in-memory database is the fastest way
+to run the tests:
+
   $ bundle exec rake test:sqlite3_mem
 
 There should be tests available for each database backend listed in the {Config