Replace unclosed opening paren with dash

exercises/12_while2.zig

@@ -1,7 +1,7 @@
 //
 // Zig 'while' statements can have an optional 'continue expression'
 // which runs every time the while loop continues (either at the
-// end of the loop or when an explicit 'continue' is invoked (we'll
+// end of the loop or when an explicit 'continue' is invoked - we'll
 // try those out next):
 //
 //     while (condition) : (continue expression) {