docs: add note about endpoint config option

[danielmbarlow]

Which problem is this PR solving?

The documentation didn't show that the URL for the endpoint config option should include /api/traces

Short description of the changes

I added an extra option to the usage documentation example

[codecov]

Codecov Report

Merging #1234 into master will increase coverage by 1.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1234      +/-   ##
==========================================
+ Coverage   92.05%   93.24%   +1.18%     
==========================================
  Files         102      122      +20     
  Lines        2367     3551    +1184     
  Branches      431      714     +283     
==========================================
+ Hits         2179     3311    +1132     
- Misses        188      240      +52     

Impacted Files	Coverage Δ
packages/opentelemetry-plugin-https/src/utils.ts	100.00% <0.00%> (ø)
...ackages/opentelemetry-exporter-zipkin/src/types.ts	100.00% <0.00%> (ø)
...ackages/opentelemetry-exporter-zipkin/src/utils.ts	100.00% <0.00%> (ø)
packages/opentelemetry-node/src/config.ts	100.00% <0.00%> (ø)
packages/opentelemetry-plugin-grpc/src/utils.ts	93.75% <0.00%> (ø)
...pentelemetry-exporter-prometheus/src/prometheus.ts	91.39% <0.00%> (ø)
...ackages/opentelemetry-exporter-jaeger/src/utils.ts	100.00% <0.00%> (ø)
...ackages/opentelemetry-exporter-jaeger/src/types.ts	100.00% <0.00%> (ø)
...es/opentelemetry-node/src/instrumentation/utils.ts	90.47% <0.00%> (ø)
packages/opentelemetry-plugin-http/src/utils.ts	97.42% <0.00%> (ø)
... and 10 more

[mayurkale22]

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