README.md: correct manual sharded testing comment
README.md: correct manual sharded testing comment

GTEST_TOTAL_SHARDS is the total count, it's GTEST_SHARD_INDEX that's 0
based.

Change-Id: I36604598873b97e4c30d5f051fdce8c6262d2e82

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom