Fix TEST_SHARD_INDEX documentation.
RELNOTES:
@@ -171,7 +171,7 @@ once per shard. The environment variable
<a href="#initial-conditions"><code>TEST_TOTAL_SHARDS</code></a> is the number
of shards, and
<a href="#initial-conditions"><code>TEST_SHARD_INDEX</code></a> is the shard
-index, beginning at 1. Runners use this information to select which tests to
+index, beginning at 0. Runners use this information to select which tests to
run - for example, using a round-robin strategy. Not all test runners support sharding.
If a runner supports sharding, it must create or update the last modified date
of the file specified by
