Fix the platform name of the windowing_test target for macOS in ci.yaml (flutter#177472)

The name of platform within the target name must exactly match one of
the platforms declared in the platform_properties section.

Fixes flutter#177465

Author: jason-simmons

.ci.yaml

@@ -7005,14 +7005,14 @@ targets:
         ["devicelab", "hostonly", "linux"]
       task_name: windowing_test_linux
 
-  - name: MacOS windowing_test
+  - name: Mac windowing_test
     recipe: devicelab/devicelab_drone
     presubmit: true
     bringup: true
     timeout: 60
     properties:
       tags: >
-        ["devicelab", "hostonly", "macos"]
+        ["devicelab", "hostonly", "mac"]
       task_name: windowing_test_macos