Update README.md

Co-authored-by: Ralf Jung <post@ralfj.de>

README.md

@@ -67,7 +67,7 @@ in your program, and cannot run all programs:
   loop forever. There is no threading support on Windows.
 * Weak memory emulation may produce weak behaivours unobservable by compiled
   programs running on real hardware when `SeqCst` fences are used, and it cannot
-  produce all possibly observable behaviours on real hardware.
+  produce all behaviors possibly observable on real hardware.
 
 [rust]: https://www.rust-lang.org/
 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md