mu-man: improve typesetting of references

Try to stick to the man-pages(7) standard.

man/muhome.inc

@@ -2,7 +2,7 @@
 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=.
+=~/.mu=, which now requires *--muhome=~/.mu*.
 
 The environment variable ~MUHOME~ can be used as an alternative to *--muhome*. The
 latter has precedence.