[DOC] Small fix for Histogram documentation.

[lalitb]

Fixes # (issue)

Changes

The comment is not consistent with implementation and specification. The Histogram API only records non-negative value.

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #2156 (a8763f7) into main (770fce3) will decrease coverage by 0.02%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2156      +/-   ##
==========================================
- Coverage   87.17%   87.15%   -0.02%     
==========================================
  Files         166      166              
  Lines        4777     4777              
==========================================
- Hits         4164     4163       -1     
- Misses        613      614       +1     

Impacted Files	Coverage Δ
...i/include/opentelemetry/metrics/sync_instruments.h	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

[marcalff]

LGTM, thanks for the fix.

[esigo]

LGTM
Thanks :)