documentation error

[Chad Smith]

The mkisofs command on
https://www.freebsd.org/doc/handbook/install-advanced.html needs a space
between where the output file name is specified and the pathspec at the
very end of the command.

Running the command as documented with proper substitutions produces an
error:
mkisofs: Missing pathspec.

Running the command with a space between the output file name and the
pathspec works without issue.