Fix ranking in spearman metric

[SkafteNicki]

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• Did you write any new necessary tests?

What does this PR do?

Fixes #444
Fix bug in the ranking function used in the spearman metric.

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[codecov]

Codecov Report

Merging #448 (07e4355) into master (18d500c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #448   +/-   ##
=======================================
  Coverage   96.00%   96.00%           
=======================================
  Files         129      129           
  Lines        4180     4180           
=======================================
  Hits         4013     4013           
  Misses        167      167           

Flag	Coverage Δ
Linux	74.73% <0.00%> (ø)
Windows	74.73% <0.00%> (ø)
cpu	95.98% <100.00%> (+0.04%)	⬆️
gpu	95.93% <100.00%> (ø)
macOS	95.98% <100.00%> (+0.04%)	⬆️
pytest	96.00% <100.00%> (ø)
python3.6	?
python3.8	95.98% <100.00%> (+0.04%)	⬆️
python3.9	?
torch1.3.1	?
torch1.4.0	95.31% <100.00%> (+0.04%)	⬆️
torch1.9.0	95.90% <100.00%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
torchmetrics/functional/regression/spearman.py	97.77% <100.00%> (ø)

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