Fix an incorrect event name.

json_reporter.md

 JSON Reporter Protocol
 ======================
 
 The test runner supports a JSON reporter which provides a machine-readable
 representation of the test runner's progress. This reporter is intended for use
 by IDEs and other tools to present a custom view of the test runner's operation
 without needing to parse output intended for humans.
 
 Note that the test runner is highly asynchronous, and users of this protocol
 shouldn't make assumptions about the ordering of events beyond what's explicitly
@@ skipping 83 matching lines @@
   String protocolVersion;
 
   // The version of the test runner being used.
   String runnerVersion;
 }
 ```
 
 A single start event is emitted before any other events. It indicates that the
 test runner has started running.
 
-### SuiteCountEvent
+### AllSuitesEvent
 
 ```
 class AllSuitesEvent {
   String type = "allSuites";
 
   /// The total number of suites that will be loaded.
   int count;
 }
 ```
 
@@ skipping 249 matching lines @@
 class Metadata {
   // Whether the test case will be skipped by the test runner.
   bool skip;
 
   // The reason the test case is skipped, if the user provided it.
   String? skipReason;
 }
 ```
 
 The metadata attached to a test by a user.