ctlv3: have "protobuf" in output help string instead of "proto"

[heyitsanthony]

Fixes #7538

[xiang90]

LGTM

[fanminshi]

lgtm

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@fba8755). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #7539   +/-   ##
=========================================
  Coverage          ?   70.22%           
=========================================
  Files             ?      320           
  Lines             ?    26214           
  Branches          ?        0           
=========================================
  Hits              ?    18409           
  Misses            ?     6351           
  Partials          ?     1454

Impacted Files	Coverage Δ
etcdctl/ctlv3/ctl.go	100% <100%> (ø)

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