Use nonnegative-integer? in favor of natural-number?

The natural-number? predicate seems to be
unavailable (though there is a remnant of it in the form of
natural-number/c, which is just a contract alias of
exact-nonnegative-integer?). In any case, it surely means
nonnegative integer.

(One might push the envelope here slightly and add
exactness, but this commit does not go that far.)

brag/brag/brag.scrbl

@@ -1042,9 +1042,9 @@ In addition to the exports shown below, the @racketmodname[brag/support] module
 @defproc[(token [type (or/c string? symbol?)]
                 [val any/c #f]
                 [#:line line (or/c positive-integer? #f) #f]
-                [#:column column (or/c natural-number? #f) #f]
+                [#:column column (or/c nonnegative-integer? #f) #f]
                 [#:position position (or/c positive-integer? #f) #f]
-                [#:span span (or/c natural-number? #f) #f]
+                [#:span span (or/c nonnegative-integer? #f) #f]
                 [#:skip? skip? boolean? #f]
                 )
          token-struct?]{
@@ -1060,9 +1060,9 @@ In addition to the exports shown below, the @racketmodname[brag/support] module
 @defstruct[token-struct ([type symbol?]
                          [val any/c]
                          [position (or/c positive-integer? #f)]
-                         [line (or/c natural-number? #f)]
+                         [line (or/c nonnegative-integer? #f)]
                          [column (or/c positive-integer? #f)]
-                         [span (or/c natural-number? #f)]
+                         [span (or/c nonnegative-integer? #f)]
                          [skip? boolean?])
            #:transparent]{
  The token structure type.