Update comment for x18 register

PiperOrigin-RevId: 315455188
diff --git a/src/f16-gemm/6x8-aarch64-neonfp16arith-ld64.S.in b/src/f16-gemm/6x8-aarch64-neonfp16arith-ld64.S.in
index aafe90d..d20a49c 100644
--- a/src/f16-gemm/6x8-aarch64-neonfp16arith-ld64.S.in
+++ b/src/f16-gemm/6x8-aarch64-neonfp16arith-ld64.S.in
@@ -21,7 +21,7 @@
 $else:
   #     const union xnn_f16_scaleminmax_params params[restrict XNN_MIN_ELEMENTS(1)])  [sp + 8] -> x8
 
-# d8-d15, x19-x30 need to be preserved if used. x18 is reserved on iOS.
+# d8-d15, x19-x30 need to be preserved if used. x18 is reserved by the OS.
 
 # A pointers
 #  x3 a0