Fix code to correspond to equation for matthews_corrcoef

[SkafteNicki]

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What does this PR do?

Fixes #320
Currently there is a small mismatch between the definition of matthews corrcoef and our code. The code was still correct because the equation is symmetrical, but as requested by an user our code should match the definition. PR fixes this.

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[codecov]

Codecov Report

Merging #321 (a0e3847) into master (aa554ac) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
+ Coverage   96.58%   96.64%   +0.05%     
==========================================
  Files         107      107              
  Lines        3428     3428              
==========================================
+ Hits         3311     3313       +2     
+ Misses        117      115       -2     

Flag	Coverage Δ
Linux	75.83% <100.00%> (ø)
Windows	75.83% <100.00%> (ø)
cpu	96.58% <100.00%> (ø)
gpu	96.55% <100.00%> (?)
macOS	96.58% <100.00%> (ø)
pytest	96.64% <100.00%> (+0.05%)	⬆️
python3.6	95.67% <100.00%> (ø)
python3.8	96.58% <100.00%> (ø)
python3.9	96.49% <100.00%> (ø)
torch1.3.1	95.67% <100.00%> (ø)
torch1.4.0	95.77% <100.00%> (ø)
torch1.9.0	96.49% <100.00%> (ø)

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Impacted Files	Coverage Δ
...ics/functional/classification/matthews_corrcoef.py	100.00% <100.00%> (ø)
torchmetrics/functional/regression/spearman.py	97.77% <0.00%> (+4.44%)	⬆️

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[Borda]

@SkafteNicki shall we add some text covering this case as you made a change but no effect on tested cases...

[SkafteNicki]

@Borda the change does not matter for the correctness of the result because both quantities are just multiplied together in the end. However, the user that raised the issue found it confusing that the two quantities where switched around (compared to the formula on Wikipedia) and therefore wanted it changed.