operator-expr: remove stray word in footnote

"only for closures that {x} can be {y}" repeats the limitation; it should
either be "only closures that {x} can be {y}", or, if the capability ("{y}") is
implicit, it can be "only for closures that {x}". Go with the first option that
keeps the context, and remove the extra "for".

Author: DanielEScherzer

src/expressions/operator-expr.md

@@ -549,7 +549,7 @@ reference types and `mut` or `const` in pointer types.
 [^lessmut]: only when `m₁` is `mut` or `m₂` is `const`. Casting `mut` reference/pointer to
 `const` pointer is allowed.
 
-[^no-capture]: only for closures that do not capture (close over) any local variables can be casted to function pointers.
+[^no-capture]: only closures that do not capture (close over) any local variables can be casted to function pointers.
 
 ### Semantics