Finish incomplete sentence in docs/design-docs/rpc-retry-and-failover.md
Finish incomplete sentence in docs/design-docs/rpc-retry-and-failover.md

This just finishes an incomplete sentence that was missed
during review.

Change-Id: I5a390d76d053dd34f20c058a1733ba949143d2b3
Reviewed-on: http://gerrit.cloudera.org:8080/5228
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

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