fix typo in assert_changes error message

activesupport/lib/active_support/testing/assertions.rb

@@ -157,7 +157,7 @@ def assert_changes(expression, message = nil, from: UNTRACKED, to: UNTRACKED, &b
         after = exp.call
 
         if to == UNTRACKED
-          error = "#{expression.inspect} didn't changed"
+          error = "#{expression.inspect} didn't change"
           error = "#{message}.\n#{error}" if message
           assert_not_equal before, after, error
         else