Execution order: Spell out uninitialized explicitly on field as well

It could have been read that it was any field, not just uninitialized fields.

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

src/expressions/operator-expr.md

@@ -427,7 +427,7 @@ operand.
 > is evaluated before the left one.
 
 It then has the effect of first [dropping] the value at the assigned place,
-unless the place is an uninitialized local variable or field of a local
+unless the place is an uninitialized local variable or an uninitialized field of a local
 variable. Next it either [copies or moves] the assigned value to the assigned
 place.