Corrects documentation error

Item 22. in cdist description had parenthetical that read "see sokalsneath function documentation", but should have read, "see wminkowski function documentation." That is fixed here.
Merge pull request #3012 from lemonlaug/master

DOC: corrects documentation error in spatial.cdist
Merge remote-tracking branch 'upstream/master' into clpmn
fixing signs in the derivative of LegendreP