Fix some variable spelling errors

[JoeWrightss]

"ErrNoAddrAvilable" to "ErrNoAddrAvailable".

[codecov-io]

Codecov Report

Merging #10531 into master will decrease coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #10531      +/-   ##
==========================================
- Coverage   71.81%   71.61%   -0.21%     
==========================================
  Files         393      393              
  Lines       36521    36521              
==========================================
- Hits        26227    26153      -74     
- Misses       8474     8534      +60     
- Partials     1820     1834      +14

Impacted Files	Coverage Δ
clientv3/balancer/grpc1.7-health.go	59.01% <100%> (ø)	⬆️
pkg/adt/interval_tree.go	83.18% <0%> (-9.61%)	⬇️
proxy/grpcproxy/watcher.go	85.71% <0%> (-8.17%)	⬇️
pkg/fileutil/purge.go	65.9% <0%> (-6.82%)	⬇️
clientv3/leasing/util.go	91.66% <0%> (-6.67%)	⬇️
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
proxy/grpcproxy/watch.go	88.55% <0%> (-4.22%)	⬇️
pkg/transport/listener.go	54.5% <0%> (-3.8%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
... and 17 more

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[jingyih]

lgtm