sysutils/u-boot-starfive-visionfive2: fix description

Some interactive shells (csh, zsh) will try to interpret the '!' at the
beginning of the partition type as a previous command. Unconditionally
escape this in the example commands, as it can be a source of confusion.

PR: 276765
Reported by: rdunkle@smallcatbrain.com
Sponsored by: The FreeBSD Foundation