Corrected BNF input documentation for fast-import.

Corrected BNF input documentation for fast-import.

Now that fast-import uses uintmax_t (the largest available unsigned
integer type) for marks we don't want to say its an unsigned 32
bit integer in ASCII base 10 notation.  It could be much larger,
especially on 64 bit systems, and especially if a frontend uses
a very large number of marks (1 per file revision on a very, very
large import).

Signed-off-by: Shawn O. Pearce <redacted>