Correct a typo in threads_win32.h

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6238>
diff --git a/include/c11/threads_win32.h b/include/c11/threads_win32.h
index 326cfc4..55e687c 100644
--- a/include/c11/threads_win32.h
+++ b/include/c11/threads_win32.h
@@ -51,7 +51,7 @@
     Max registerable TSS dtor number.
 */
 
-// XXX: Retain XP compatability
+// XXX: Retain XP compatibility
 #if 0
 #if _WIN32_WINNT >= 0x0600
 // Prefer native WindowsAPI on newer environment.