docs(prometheus-exporter): fix prometheus exporter examples in README

[legendecas]

Which problem is this PR solving?

Fixes #3107

Type of change

• This change requires a documentation update

Checklist:

• Followed the style guidelines of this project
• Documentation has been updated

[codecov]

Codecov Report

Merging #3110 (3be670e) into main (43f4e5a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3110   +/-   ##
=======================================
  Coverage   93.20%   93.20%           
=======================================
  Files         196      196           
  Lines        6431     6431           
  Branches     1359     1359           
=======================================
  Hits         5994     5994           
  Misses        437      437           

Impacted Files	Coverage Δ
...-trace-base/src/platform/node/RandomIdGenerator.ts	87.50% <0.00%> (-6.25%)	⬇️
...emetry-core/src/platform/node/RandomIdGenerator.ts	93.75% <0.00%> (+6.25%)	⬆️

[legendecas]

@open-telemetry/javascript-approvers would you mind taking a look at this PR? thank you!