Fix a typo in apis.md

sky/specs/apis.md

 APIS
 ====
 
 The Sky core API
 ----------------
 
 ```javascript
 module 'sky:core' {
 
   // EVENTS
@@ skipping 35 matching lines @@
   typedef ChildArgument (Element or Text or String);
 
   abstract interface Node : EventTarget {
     readonly attribute TreeScope? ownerScope; // O(1)
     
     readonly attribute ParentNode? parentNode; // O(1)
     readonly attribute Element? parentElement; // O(1) // if parentNode isn't an element, returns null
     readonly attribute ChildNode? previousSibling; // O(1)
     readonly attribute ChildNode? nextSibling; // O(1)
     
-    // the following all throw is parentNode is null
+    // the following all throw if parentNode is null
     void insertBefore(ChildArgument... nodes); // O(N) in number of arguments plus all their descendants
     void insertAfter(ChildArgument... nodes); // O(N) in number of arguments plus all their descendants
     void replaceWith(ChildArgument... nodes); // O(N) in number of descendants plus arguments plus all their descendants
     void remove(); // O(N) in number of descendants
     Node cloneNode(Boolean deep); // O(1) if deep=false, O(N) in the number of descendants if deep=true
 
     // called when parentNode changes
     virtual void parentChangeCallback(ParentNode? oldParent, ParentNode? newParent, ChildNode? previousSibling, ChildNode? nextSibling); // O(N) in descendants (calls attached/detached)
     virtual void attachedCallback(); // noop
     virtual void detachedCallback(); // noop
@@ skipping 240 matching lines @@
 magical imports:
   the core mojo fabric JS API   sky:mojo:fabric:core
   the asyncWait/cancelWait mojo fabric JS API (interface to IPC thread)  sky:mojo:fabric:ipc
   the mojom for the shell, proxying through C++ so that the shell pipe isn't exposed  sky:mojo:shell
   the sky API  sky:core
 ```
 
 TODO(ianh): determine if we want to separate the "this" from the
 Document, especially for Modules, so that exposing a module's element
 doesn't expose the module's exports attribute.