Fix spelling error in ActiveModel::Conversion docs. [#4826 state:resolved]

activemodel/lib/active_model/conversion.rb

@@ -30,7 +30,7 @@ def to_model
       self
     end
 
-    # Returns an Enumerable of all (primary) key attributes or nil if persisted? is fakse
+    # Returns an Enumerable of all (primary) key attributes or nil if persisted? is false
     def to_key
       persisted? ? [id] : nil
     end