generator: Use %g format for g_ascii_formatd()

I should read the documentation for the functions I use:

  Converts a gdouble to a string, using the '.' as decimal point. To
  format the number you pass in a printf()-style format string. Allowed
  conversion specifiers are 'e', 'E', 'f', 'F', 'g' and 'G'.
                -- from g_ascii_formatd() in GLib's API reference

Epic reading fail.