Fix README badge URLs

[irnnr]

URLs for README badges related to required Node.js version and npm package site were pointing to the Prometheus exporter. This changes the URLs to point to the correct package.

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[irnnr]

update: Signed the CLA.

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[codecov-io]

Codecov Report

Merging #509 into master will increase coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
+ Coverage   94.61%   94.93%   +0.32%     
==========================================
  Files         145      145              
  Lines       10172    10074      -98     
  Branches      883      883              
==========================================
- Hits         9624     9564      -60     
+ Misses        548      510      -38

Impacted Files	Coverage Δ
test/test-instana.ts	61.11% <0%> (-24.08%)	⬇️
src/common-utils.ts	95.83% <0%> (-0.84%)	⬇️
test/test-ioredis.ts	97.16% <0%> (-0.55%)	⬇️
src/binary-format.ts	100% <0%> (ø)	⬆️
src/grpc-stats/client-stats.ts	100% <0%> (ø)	⬆️
src/grpc-stats/common-distributions.ts	100% <0%> (ø)	⬆️
src/grpc-stats/stats-common.ts	100% <0%> (ø)	⬆️
src/validators.ts	100% <0%> (ø)	⬆️
test/test-detect-resource.ts	99.51% <0%> (+0.37%)	⬆️
test/test-jaeger-format.ts	97.18% <0%> (+1.79%)	⬆️
... and 3 more

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[mayurkale22]

Thanks for the PR.