reference: Switch wording to "closure can be called once" for FnOnce

Author: Ulrik Sverdrup

src/doc/reference.md

@@ -3616,7 +3616,7 @@ Depending on the requirements of the closure, its type implements one or
 more of the closure traits:
 
 * `FnOnce`
-  : The closure can be called at least once. A closure called as `FnOnce`
+  : The closure can be called once. A closure called as `FnOnce`
     can move out values from its environment.
 
 * `FnMut`