docs: update moved secret token link

[bmorelli25]

For elastic/apm-server#2728. Must be merged at the same time.

[bmorelli25]

ci/docs failure is expected as elastic/apm-server#2728 hasn't been merged yet.

[vigneshshanmugam]

Thanks @bmorelli25

[codecov-io]

Codecov Report

Merging #431 into master will decrease coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
- Coverage   94.55%   94.39%   -0.17%     
==========================================
  Files          40       39       -1     
  Lines        2058     1767     -291     
  Branches      442      350      -92     
==========================================
- Hits         1946     1668     -278     
+ Misses        109       96      -13     
  Partials        3        3

Impacted Files	Coverage Δ
.../src/performance-monitoring/transaction-service.js	93.12% <0%> (-1.71%)	⬇️
...rum-core/src/performance-monitoring/transaction.js	98.61% <0%> (-1.39%)	⬇️
packages/rum/src/apm-base.js	92.78% <0%> (-0.43%)	⬇️
...c/performance-monitoring/performance-monitoring.js	95.02% <0%> (-0.36%)	⬇️
packages/rum-core/src/common/utils.js	95.12% <0%> (-0.26%)	⬇️
packages/rum-core/src/common/config-service.js	96.73% <0%> (-0.09%)	⬇️
...s/rum-core/src/performance-monitoring/breakdown.js
packages/rum-core/src/common/apm-server.js	97.63% <0%> (+3.66%)	⬆️