update bash-completion for zipf argument

Signed-off-by: Su Lifan <su-lifan@linux.alibaba.com>

Author: lfsu-ali

bash-completion/memtier_benchmark

@@ -27,7 +27,7 @@ _memtier_completions()
 
   options_no_args=("--debug" "--show-config" "--hide-histogram" "--distinct-client-seed" "--randomize"\
                    "--random-data" "--data-verify" "--verify-only" "--generate-keys" "--key-stddev"\
-                   "--key-median" "--no-expiry" "--cluster-mode" "--help" "--version"\
+                   "--key-median" "--key-zipf-exp" "--no-expiry" "--cluster-mode" "--help" "--version"\
                    "-D" "-R" "-h" "-v")
 
   options_comp=("--protocol" "-P" "--key-pattern" "--data-size-pattern" "--command-key-pattern")