Check: correct the memory limit

[spzala]

We should have limit of 3M not 30 as it's said in the doc comment.

[codecov-io]

Codecov Report

Merging #9806 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #9806      +/-   ##
=========================================
+ Coverage   69.58%   69.6%   +0.01%     
=========================================
  Files         376     376              
  Lines       35229   35229              
=========================================
+ Hits        24514   24521       +7     
+ Misses       8954    8943      -11     
- Partials     1761    1765       +4

Impacted Files	Coverage Δ
etcdctl/ctlv3/command/check.go	14% <ø> (ø)	⬆️
mvcc/watcher.go	96.29% <0%> (-3.71%)	⬇️
proxy/httpproxy/director.go	80% <0%> (-2.86%)	⬇️
raft/progress.go	94.17% <0%> (-1.95%)	⬇️
etcdserver/api/rafthttp/msgappv2_codec.go	69.56% <0%> (-1.74%)	⬇️
pkg/adt/interval_tree.go	87.08% <0%> (-1.51%)	⬇️
mvcc/watchable_store.go	82.8% <0%> (-1.41%)	⬇️
etcdserver/api/v2http/client.go	85.51% <0%> (-1.21%)	⬇️
etcdserver/apply.go	88.68% <0%> (-0.34%)	⬇️
etcdserver/server.go	73.05% <0%> (-0.08%)	⬇️
... and 11 more

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[xiang90]

lgtm