[ISSUE #694] Fix typos.

[wayilau]

What is the purpose of the change

Fix typos.

Brief changelog

fecth->fetch.

[codecov-commenter]

Codecov Report

Merging #689 (dd21e70) into master (c688c19) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #689   +/-   ##
=======================================
  Coverage   24.71%   24.71%           
=======================================
  Files          48       48           
  Lines        5223     5223           
=======================================
  Hits         1291     1291           
  Misses       3726     3726           
  Partials      206      206           

Impacted Files	Coverage Δ
consumer/offset_store.go	65.09% <50.00%> (ø)

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[wenfengwang]

LGTM

#694