mu-man: typeset option values differently

Use underlining only, do not use <>, as it is the most often done.

man/muhome.inc

@@ -1,5 +1,5 @@
 ** --muhome
-use a non-default directory to store and read the database, write the logs, etc.
+Use a non-default directory to store and read the database, write the logs, etc.
 By default, *mu* uses the XDG Base Directory Specification (e.g. on GNU/Linux this
 defaults to _~/.cache/mu_ and _~/.config/mu_). Earlier versions of *mu* defaulted to
 _~/.mu_, which now requires *--muhome=~/.mu*.