libstdc++/4532: typo in bits/stl_algo.h for partial_sort

[Phil Edwards]

> http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view&pr=4532&database=gcc

This fix is already in my local tree as part of PR 3666 (and others).
As soon as my concept checking patch (see archives) is approved, this will
be one of the many fixes checked in.

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