util: Correct some help text for the initparam m5 utility command.
util: Correct some help text for the initparam m5 utility command.

The "key" value passed to the initparam command can be at most 16
characters long, but the text said it had to be shorter than 16
characters.

Change-Id: Ic98dc87feb868b44c4210ae16cb5b0dad2a67d88
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27227
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5