amendment to factor pgm

[Arnim Littek]

Entering a parameter larger than 2^64 to the program "factor" causes
an error message which tells me the input is not a valid positive
integer, and then refers me to the --help message.

Somewhere, the program wants to have a reasonable limit, so the help
message probably wants to say the factor package is limited to
integers <= 2^64, rather than the grandiose statement that an overly
large input is not a valid positive integer.

Arnim.