Add example to parts documentation [ci-skip]

Author: m-nakamura145

activesupport/lib/active_support/duration.rb

@@ -234,7 +234,10 @@ def initialize(value, parts, variable = nil) # :nodoc:
       end
     end
 
-    # Returns a copy of the parts hash that defines the duration
+    # Returns a copy of the parts hash that defines the duration.
+    #
+    #   5.minutes.parts # => {:minutes=>5}
+    #   3.years.parts # => {:years=>3}
     def parts
       @parts.dup
     end