Update comment for x18 register

PiperOrigin-RevId: 315455188
diff --git a/src/f32-gemm/1x8-aarch64-neonfma-cortex-a53.S.in b/src/f32-gemm/1x8-aarch64-neonfma-cortex-a53.S.in
index 338fa48..d503530 100644
--- a/src/f32-gemm/1x8-aarch64-neonfma-cortex-a53.S.in
+++ b/src/f32-gemm/1x8-aarch64-neonfma-cortex-a53.S.in
@@ -21,7 +21,7 @@
 $else:
   #     const union xnn_f32_minmax_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 pointer
 # x3  a0