Update operator-expr.md

Author: joshlf

src/expressions/operator-expr.md

@@ -480,7 +480,7 @@ assert_eq!(values[1], 3);
 
 #### Slice DST pointer to pointer cast
 
-When `T` and `U` are both "slice DSTs" - ie, slice types or types whose trailing field
+When `T` and `U` are both "slice DSTs" - i.e., slice types or types whose trailing field
 is a slice type - the raw pointer types `*const T`, `*mut T`, `*const U`, and `*mut U`
 encode the number of elements in this slice. Casts between these raw pointer types
 preserve the number of elements. Note that, as a consequence, such casts do *not*