Fix documentation error for nc chi-squared dist

Fix documentation error for the PDF of the non-central chi-squared continuous
distribution, which was described wrong but implemented correctly. Also do some
minor style fixes (remove spaces at line end).
Merge remote-tracking branch 'upstream/master' into chi2-documentation-fix
Merge pull request #4145 from LifeIsHealthy/chi2-documentation-fix

DOC: Fix documentation error for nc chi-squared dist