fixing the chi2 value printed in MFLike

[sgiardie]

In the line where we print the logp and the chi2, we have

chi2 = self._fast_chi_squared(self.inv_cov, delta)
logp = -0.5 * chi2 + self.logp_const
self.log.debug(f"Log-likelihood value computed = {logp} (Χ² = {-2 * chi2})")

so we print -2 chi2 instead of chi2. The extra -2 is a leftover factor from a previous implementation, see the version before this commit.
So I just removed the extra -2

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.45%. Comparing base (894e3f3) to head (5f15256).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #96   +/-   ##
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  Coverage   85.45%   85.45%           
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  Files           3        3           
  Lines         550      550           
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  Hits          470      470           
  Misses         80       80           

Files with missing lines	Coverage Δ
mflike/mflike.py	83.08% <100.00%> (ø)