Fix misptint

Injected at dab6c3aa

guides/source/contributing_to_ruby_on_rails.textile

@@ -211,7 +211,7 @@ will now run the four of them in turn.
 You can also run any single test separately:
 
 <shell>
-$ ARRCONN=sqlite3 ruby -Itest test/cases/associations/has_many_associations_test.rb
+$ ARCONN=sqlite3 ruby -Itest test/cases/associations/has_many_associations_test.rb
 </shell>
 
 You can invoke +test_jdbcmysql+, +test_jdbcsqlite3+ or +test_jdbcpostgresql+ also. See the file +activerecord/RUNNING_UNIT_TESTS+ for information on running more targeted database tests, or the file +ci/travis.rb+ for the test suite run by the continuous integration server.