x86/topology: Fix function name in documentation

topology_sibling_cpumask() is the correct thread-related topology
function in the kernel:

  s/topology_sibling_mask/topology_sibling_cpumask
Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: linux-doc@vger.kernel.org
Link: http://lkml.kernel.org/r/20180222084812.14497-1-douly.fnst@cn.fujitsu.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/x86/topology.txt

@@ -108,7 +108,7 @@ The topology of a system is described in the units of:
 
     The number of online threads is also printed in /proc/cpuinfo "siblings."
 
-  - topology_sibling_mask():
+  - topology_sibling_cpumask():
 
     The cpumask contains all online threads in the core to which a thread
     belongs.