docs: Adding example to assert_difference method

Just found out that `assert_difference` supports passing a hash of
expressions/numeric differences, so, I thought it'd be a good idea to
include it as an example.

Example:
```ruby
assert_difference({ 'Article.count' => 1, 'Notification.count' => 2 }) do
  post :create, params: { article: {...} }
end
```

Author: EduardoGHdez

activesupport/lib/active_support/testing/assertions.rb

@@ -71,19 +71,19 @@ def assert_nothing_raised
       #     post :delete, params: { id: ... }
       #   end
       #
-      # An array of expressions can also be passed in and evaluated.
+      # An array of expressions can be passed in and evaluated.
       #
       #   assert_difference [ 'Article.count', 'Post.count' ], 2 do
       #     post :create, params: { article: {...} }
       #   end
       #
-      # A hash of expressions/numeric differences can also be passed in and evaluated.
+      # A hash of expressions/numeric differences can be passed in and evaluated.
       #
-      #   assert_difference ->{ Article.count } => 1, ->{ Notification.count } => 2 do
+      #   assert_difference({ 'Article.count' => 1, 'Notification.count' => 2 }) do
       #     post :create, params: { article: {...} }
       #   end
       #
-      # A lambda or a list of lambdas can be passed in and evaluated:
+      # A lambda, a list of lambdas or a hash of lambdas/numeric differences can be passed in and evaluated:
       #
       #   assert_difference ->{ Article.count }, 2 do
       #     post :create, params: { article: {...} }
@@ -93,6 +93,10 @@ def assert_nothing_raised
       #     post :create, params: { article: {...} }
       #   end
       #
+      #   assert_difference ->{ Article.count } => 1, ->{ Notification.count } => 2 do
+      #     post :create, params: { article: {...} }
+      #   end
+      #
       # An error message can be specified.
       #
       #   assert_difference 'Article.count', -1, 'An Article should be destroyed' do