close.2, poll.2, pthread_spin_init.3: Remove section number in page self-references

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

man2/close.2

@@ -158,7 +158,7 @@ until the I/O system call completes.
 for a discussion of open file descriptions.)
 Thus, the blocking system call in the first thread may successfully
 complete after the
-.BR close (2)
+.BR close ()
 in the second thread.
 .\"
 .SS Dealing with error returns from close()

man2/poll.2

@@ -52,7 +52,7 @@ to perform I/O.
 The Linux-specific
 .BR epoll (7)
 API performs a similar task, but offers features beyond those found in
-.BR poll (2).
+.BR poll ().
 .PP
 The set of file descriptors to be monitored is specified in the
 .I fds

man3/pthread_spin_init.3

@@ -102,7 +102,7 @@ The result of performing operations such as
 .BR pthread_spin_lock (3),
 .BR pthread_spin_unlock (3),
 and
-.BR pthread_spin_destroy (3)
+.BR pthread_spin_destroy ()
 on
 .I copies
 of the object referred to by