Fix: specification: tag IDs first, then variable values second

[GeorgeHoneywood]

Hey! I've been writing my own parser for Mapsforge map files and I think the current wording around variable tag values is a little confusing.

The specification docs/Specification-Binary-Map-File.md makes it sound like tag IDs and their variable values are interleaved. For example something like tag_id_1, variable_value_1, tag_id_2, variable_value_2.

for each tag of the POI:

    tag id as VBE-U INT
    variable values as different data types, whose content can be evaluated from tag's wildcard

In reality, it seems like files are written with all the tag IDs first, then all the tag values second, like tag_id_1, tag_id_2, variable_value_1, variable_value_2. See here for the implementation in mapsforge-map-reader.

I'm not entirely sure about how I've changed the wording, but I think it helps clear it up.

Thanks for working on this great project!

[devemux86]

Thanks!