Clarify the ministat default width

The man page states that:
'-w width Width of ASCII-art plot in characters, default is 74.'

This is not entirely correct. The mini-help is more accurate:
'-w : width of graph/test output (default 74 or terminal width)'

In other words: the man page fails to explain that ministat will default
to the terminal width, not 74. It will only fall back to 74 if stdout is
not a TTY.

Submitted by: Ben Hutchings <ben@decadent.org.uk>
Approved by: philip (mentor)