fix grammar

Co-authored-by: Ryan Scheel <[email protected]>

Author: RalfJung

src/behavior-considered-undefined.md

@@ -59,7 +59,7 @@ code.
 
 **Note:** Uninitialized memory is also implicitly invalid for any type that has
 a restricted set of valid values. In other words, the only cases in which
-reading uninitialized memory is permitted is inside `union`s and in "padding"
+reading uninitialized memory is permitted are inside `union`s and in "padding"
 (the gaps between the fields/elements of a type).
 
 A reference/pointer is "dangling" if it is null or not all of the bytes it