Fixing section heading markup I probably borked when adding ID

Author: hackermatthew

ch03.asciidoc

@@ -663,7 +663,8 @@ While we've already addressed syntax enhancements coming to object literals in <
 
 We've already looked at `Object.getOwnPropertySymbols`, but let's also take a look at `Object.assign`, `Object.is`, and `Object.setPrototypeOf`.
 
-[[extending_objects_with_object_assign]]==== 3.3.1 Extending objects with `Object.assign`
+[[extending_objects_with_object_assign]]
+==== 3.3.1 Extending objects with `Object.assign`
 
 The need to provide default values for a configuration object is not at all uncommon. Typically, libraries and well-designed component interfaces come with sensible defaults that cater to the most frequented use cases.