clone.2: Small rewording of explanation of clone() wrt threads

Clone has so many effects that it's an oversimplification to say
that the *main* use of clone is to create a thread.  (In fact,
the use of clone() to create new processes may well be more
common, since glibc's fork() is a wrapper that calls clone().)

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

man2/clone.2

@@ -104,7 +104,7 @@ But see the description of
 .B CLONE_PARENT
 below.)
 
-The main use of
+One use of
 .BR clone ()
 is to implement threads: multiple threads of control in a program that
 run concurrently in a shared memory space.