Fix "default" named style mapping for filetypes.conf

"default" was mapped to "value" which is normally a string-like
style rather than a "default" type of style which make some themes
that set different background colour for strings to look weird for
config files highlighting.

data/filetypes.conf

 # For complete documentation of this file, please see Geany's main documentation
 [styling]
 # Edit these in the colorscheme .conf file instead
-default=value
+default=default
 comment=comment
 section=tag
 key=attribute