kconfig: qconf: remove non-functional href="m..." tag

The only functional tag is href="s<symbol_name>".

Commit c4f7398b ("kconfig: qconf: make debug links work again")
changed prop->name to sym->name for this reference, but it missed to
change the tag "m" to "s".

This tag is not functional at all.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>