Fix code alignment in duplicable? guide.

[skip ci]

guides/source/active_support_core_extensions.md

@@ -157,9 +157,9 @@ Active Support provides `duplicable?` to programmatically query an object about
 
 ```ruby
 "foo".duplicable? # => true
-"".duplicable?     # => true
+"".duplicable?    # => true
 0.0.duplicable?   # => false
-false.duplicable?  # => false
+false.duplicable? # => false
 ```
 
 By definition all objects are `duplicable?` except `nil`, `false`, `true`, symbols, numbers, class, and module objects.