Fix a typo in the service protocol file.

runtime/vm/service/service.md

 # Dart VM Service Protocol 3.0
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes of _version 3.0_ of the Dart VM Service Protocol. This
 protocol is used to communicate with a running Dart Virtual Machine.
 
 To use the Service Protocol, start the VM with the *--observe* flag.
 The VM will start a webserver which services protocol requests via WebSocket.
 It is possible to make HTTP (non-WebSocket) requests,
@@ skipping 40 matching lines @@
         - [Class](#class)
         - [ClassList](#classlist)
         - [Code](#code)
         - [CodeKind](#codekind)
         - [Context](#context)
         - [ContextElement](#contextelement)
         - [Error](#error)
         - [ErrorKind](#errorkind)
         - [Event](#event)
         - [EventKind](#eventkind)
-  - [ExtensionData](#extensiondata)
+        - [ExtensionData](#extensiondata)
         - [Field](#field)
         - [Flag](#flag)
         - [FlagList](#flaglist)
         - [Frame](#frame)
         - [Function](#function)
         - [Instance](#instance)
         - [Isolate](#isolate)
         - [Library](#library)
         - [LibraryDependency](#librarydependency)
         - [MapAssociation](#mapassociation)
@@ skipping 1507 matching lines @@
 
   // The fields of this Instance.
   BoundField[] fields [optional];
 
   // The elements of a List instance.
   //
   // Provided for instance kinds:
   //   List
   (@Instance|Sentinel)[] elements [optional];
 
-  // The elements of a List instance.
+  // The elements of a Map instance.
   //
   // Provided for instance kinds:
   //   Map
   MapAssociation[] associations [optional];
 
   // The bytes of a TypedData instance.
   //
   // The data is provided as a Base64 encoded string.
   //
   // Provided for instance kinds:
@@ skipping 713 matching lines @@
 ## Revision History
 
 version | comments
 ------- | --------
 1.0 | initial revision
 2.0 | Describe protocol version 2.0.
 3.0 | Describe protocol version 3.0.  Added UnresolvedSourceLocation.  Added Sentinel return to getIsolate.  Add AddedBreakpointWithScriptUri.  Removed Isolate.entry. The type of VM.pid was changed from string to int.  Added VMUpdate events.  Add offset and count parameters to getObject() and offset and count fields to Instance. Added ServiceExtensionAdded event.
 
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss