powerpc: Fixing a cputhread code documentation

--
Signed-off-by: NAnshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/cputhreads.h

@@ -9,7 +9,7 @@
  * Note: This implementation is limited to a power of 2 number of
  * threads per core and the same number for each core in the system
  * (though it would work if some processors had less threads as long
- * as the CPU numbers are still allocated, just not brought offline).
+ * as the CPU numbers are still allocated, just not brought online).
  *
  * However, the API allows for a different implementation in the future
  * if needed, as long as you only use the functions and not the variables