Fix the other microbench schema

BUG=skia:
R=bensong@google.com, jcgregorio@google.com

Author: kelvinly@google.com

Review URL: https://codereview.chromium.org/420723004

bench/microbench.json

@@ -25,7 +25,7 @@
             "type": "STRING"
         },
         {
-            "name": "isTrybot",
+            "name": "is_trybot",
             "type": "BOOLEAN"
         },
         {