doc: the relevant propert for char-numberic? is "Numeric_Type"

R6RS seems to get ths slightly wrong, since there isn't a "Numeric"
property, but a "Numeric_Type" property.

Relevant to #3460

pkgs/racket-doc/scribblings/reference/chars.scrbl

@@ -187,8 +187,8 @@ Returns @racket[#t] if @racket[char]'s Unicode general category is
 
 @defproc[(char-numeric? [char char?]) boolean?]{
 
-Returns @racket[#t] if @racket[char] has the Unicode ``Numeric''
-property.}
+Returns @racket[#t] if @racket[char] has a Unicode ``Numeric_Type''
+property value that is not @litchar{None}.}
 
 @defproc[(char-symbolic? [char char?]) boolean?]{