Update dead links

The following links are dead:

- "Let's check the [OSMPBF definition for PrimitiveBlock](https://github.com/scrosby/OSM-binary/blob/master/src/osmformat.proto#L97)."
- "What's the first byte of the data? x12, our new friend. Since we know this a PrimitiveGroup, we can look at its [OSMPBF definition](https://github.com/scrosby/OSM-binary/blob/master/src/osmformat.proto#L116) to figure out what kind of field it is."
- "Let's take a look at the entire [OSMPBF specification](https://github.com/scrosby/OSM-binary/blob/master/src/osmformat.proto#L216) for DenseNodes, because they're a bit tricky."

Could you please update them ?

Thank you :)

Find the tutorial pages containing the three quoted OSMPBF links. Check the current OSMPBF definitions for PrimitiveBlock, PrimitiveGroup, and DenseNodes, then replace each dead URL with its current location. Done means all three links resolve and still point to the definitions described in the surrounding text.

Written by the indexing model from the issue text.