fix(docs): link to correct gaxOptions docs

[stephenplusplus]

Fixes #990

[codecov]

Codecov Report

Merging #999 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #999      +/-   ##
==========================================
+ Coverage   95.17%   95.21%   +0.03%     
==========================================
  Files          22       22              
  Lines       10615    10615              
  Branches      466      509      +43     
==========================================
+ Hits        10103    10107       +4     
+ Misses        509      505       -4     
  Partials        3        3              

Impacted Files	Coverage Δ
src/iam.ts	100.00% <100.00%> (ø)
src/pubsub.ts	99.50% <100.00%> (+0.16%)	⬆️
src/subscription.ts	100.00% <100.00%> (ø)
src/topic.ts	98.63% <100.00%> (ø)
src/subscriber.ts	99.79% <0.00%> (+0.20%)	⬆️
src/message-stream.ts	98.70% <0.00%> (+0.25%)	⬆️

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[feywind]

I'm not sure why we've got all these manually specified links in the source :S ... seems like something we ought to fix. But thank you for putting in this update!