Fix typo in 2.12.1.md

I hope my next contribution is more substantial. :)

Author: rossabaker

notes/2.12.1.md

@@ -219,7 +219,7 @@ inexhaustive matches against unsealed traits.
 The following will now warn that the match might not be exhaustive,
 
 ```scala
-def nonExhautiveIfWeAssumeGuardsFalse(x: Option[Int]): Int = x match {
+def nonExhaustiveIfWeAssumeGuardsFalse(x: Option[Int]): Int = x match {
   case Some(n) if n % 2 == 0 => n
   case None => 0
 }