docs: Fixed typo in the docstrings of metrics

[fg-mindee]

This PR simply fixes a typo in the math formula of the metric expression in docstrings.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #664 (73d1e7d) into main (e14645a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #664   +/-   ##
=======================================
  Coverage   96.15%   96.15%           
=======================================
  Files         114      114           
  Lines        4447     4447           
=======================================
  Hits         4276     4276           
  Misses        171      171           

Flag	Coverage Δ
unittests	96.15% <ø> (ø)

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Impacted Files	Coverage Δ
doctr/utils/metrics.py	97.40% <ø> (ø)

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[charlesmindee]

Thanks!