Fix wrong metric description (copy-past from previous)

[pohlm01]

Apparently, the description of time_source as been copied from ptp_timescale, the metric one above. I tried to come up with a meaningful description based on Section 8.2.4.9 of the IEEE Std 1588-2019. Feel free to adopt it if necessary.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 61.58%. Comparing base (758aa8b) to head (adbb8dd).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
statime-linux/src/metrics/format.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #618   +/-   ##
=======================================
  Coverage   61.58%   61.58%           
=======================================
  Files          62       62           
  Lines        8415     8415           
=======================================
  Hits         5182     5182           
  Misses       3233     3233           

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