select.2: ffix

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/select.2

@@ -321,6 +321,7 @@ descriptor sets (that is, the total number of bits that are set in
 .IR writefds ,
 .IR exceptfds )
 which may be zero if the timeout expires before anything interesting happens.
+.PP
 On error, \-1 is returned, and
 .I errno
 is set to indicate the error;