Fix typo maxOS -> macOS

Bug: dawn:1447
Change-Id: Ic370900d2675d80a537c40c58f5ea91fe4072a60
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/92340
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>

src/dawn/native/metal/BufferMTL.mm

@@ -50,7 +50,7 @@ uint64_t Buffer::QueryMaxBufferLength(id<MTLDevice> mtlDevice) {
         return 1024 * 1024 * 1024;
     }
     // 10.11 has a 256Mb limit
-    if (@available(maxOS 10.11, *)) {
+    if (@available(macOS 10.11, *)) {
         return 256 * 1024 * 1024;
     }
 #else