RELEASES.md: ? is one of three Kleene operators

The slash and quotes in ?/“Kleene” appeared to define “Kleene” as the
name for the ? operator, which is not the case.  Rust has three Kleene
operators *, +, ?.

(Pointed out by /u/Sharlinator on Reddit.)

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

RELEASES.md

@@ -22,7 +22,7 @@ Language
 - [You can now use `_` as an identifier for consts.][61347] e.g. You can write
   `const _: u32 = 5;`.
 - [You can now use `#[repr(align(X)]` on enums.][61229]
-- [The  `?`/_"Kleene"_ macro operator is now available in the
+- [The  `?` Kleene macro operator is now available in the
   2015 edition.][60932]
 
 Compiler