doc: use correct manual references for isprint and isgraph.
doc: use correct manual references for isprint and isgraph.

* ext/posix/posix.c (isgraph, isprint): Use correct manual page
references.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

rrthomas/luaposix,snegovick/luaposix,ardje/luaposix,zevv/luaposix,jimmyharris/luaposix,luaposix/luaposix,zevv/luaposix,snegovick/luaposix,jimmyharris/luaposix,ardje/luaposix,rrthomas/luaposix