docs(README): update link to @opentelemetry/api package

[nvenegas]

Which problem is this PR solving?

• https://github.com/open-telemetry/opentelemetry-js#api link to @opentelemetry/api 404s

Short description of the changes

• Update URL in README

[johnbley]

Thanks for catching this!

[obecny]

lgtm

[codecov]

Codecov Report

Merging #2263 (cd43568) into main (4a1f2e5) will increase coverage by 0.42%.
The diff coverage is n/a.

❗ Current head cd43568 differs from pull request most recent head 394b14e. Consider uploading reports for the commit 394b14e to get more accurate results

@@            Coverage Diff             @@
##             main    #2263      +/-   ##
==========================================
+ Coverage   92.29%   92.72%   +0.42%     
==========================================
  Files         127      144      +17     
  Lines        4205     5179     +974     
  Branches      857     1060     +203     
==========================================
+ Hits         3881     4802     +921     
- Misses        324      377      +53     

Impacted Files	Coverage Δ
packages/opentelemetry-web/src/utils.ts	94.80% <0.00%> (ø)
...ation-xml-http-request/src/enums/AttributeNames.ts	100.00% <0.00%> (ø)
...kages/opentelemetry-exporter-collector/src/util.ts	100.00% <0.00%> (ø)
...emetry-instrumentation-xml-http-request/src/xhr.ts	96.58% <0.00%> (ø)
...kages/opentelemetry-web/src/StackContextManager.ts	97.05% <0.00%> (ø)
...-instrumentation-fetch/src/enums/AttributeNames.ts	100.00% <0.00%> (ø)
...ackages/opentelemetry-web/src/WebTracerProvider.ts	100.00% <0.00%> (ø)
...lemetry-exporter-collector/src/transformMetrics.ts	95.71% <0.00%> (ø)
...ages/opentelemetry-exporter-collector/src/types.ts	100.00% <0.00%> (ø)
...es/opentelemetry-context-zone-peer-dep/src/util.ts	100.00% <0.00%> (ø)
... and 7 more

[nvenegas]

Thanks, all!