Merge pull request #27791 from kenta-s/fix-sample-code-for-expand_cache_key-usage

Fix sample code for `expand_cache_key` usage [ci skip]

activesupport/lib/active_support/cache.rb

@@ -71,8 +71,8 @@ def lookup_store(*store_option)
       # each of elements in the array will be turned into parameters/keys and
       # concatenated into a single key. For example:
       #
-      #   expand_cache_key([:foo, :bar])               # => "foo/bar"
-      #   expand_cache_key([:foo, :bar], "namespace")  # => "namespace/foo/bar"
+      #   ActiveSupport::Cache.expand_cache_key([:foo, :bar])               # => "foo/bar"
+      #   ActiveSupport::Cache.expand_cache_key([:foo, :bar], "namespace")  # => "namespace/foo/bar"
       #
       # The +key+ argument can also respond to +cache_key+ or +to_param+.
       def expand_cache_key(key, namespace = nil)