[docs] fix typo in tablet-history-gc.md
[docs] fix typo in tablet-history-gc.md

Change-Id: I5a2f62d2d8e676f2fb151897daa1b7c8a3900831
Reviewed-on: http://gerrit.cloudera.org:8080/15135
Tested-by: Kudu Jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu