raft: s/leaner/learner/g

[nvanbenschoten]

"Learner" was spelled incorrectly in a few places.

[codecov-io]

Codecov Report

Merging #9088 into master will decrease coverage by 0.17%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #9088      +/-   ##
==========================================
- Coverage   76.15%   75.98%   -0.18%     
==========================================
  Files         359      359              
  Lines       29953    29953              
==========================================
- Hits        22811    22759      -52     
- Misses       5565     5604      +39     
- Partials     1577     1590      +13

Impacted Files	Coverage Δ
raft/raft.go	91.54% <0%> (ø)	⬆️
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
auth/simple_token.go	87.03% <0%> (-6.49%)	⬇️
clientv3/namespace/watch.go	93.93% <0%> (-6.07%)	⬇️
etcdctl/ctlv3/command/lease_command.go	65.34% <0%> (-5.95%)	⬇️
etcdserver/api/v3rpc/lease.go	80.88% <0%> (-5.89%)	⬇️
etcdserver/api/v3rpc/watch.go	84.01% <0%> (-5.48%)	⬇️
clientv3/concurrency/session.go	88.63% <0%> (-4.55%)	⬇️
clientv3/leasing/cache.go	87.22% <0%> (-4.45%)	⬇️
proxy/grpcproxy/watch.go	90.06% <0%> (-3.11%)	⬇️
... and 11 more

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[xiang90]

lgtm