Change from ~ to -- convention for internal symbols.

This is more common and even recommended in the recent elisp manual.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Function `weather-metno-kill-forecast-buffer' is not internal.

If it's bound to a key and accessible through the menu it should not be
considered internal.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>