gt format documentation: String prefix type should be uint64_t

I did not look up the line number since git lab reports a Error 500 (encoding error) when I try to look at doc/gt_format.rst.

This sentence in the gt format doc: For graph properties only one value follows. Strings and vectors are encoded with a length prefix of 8 bytes (uint8_t) followed by a sequence of that size with the appropriate element size.

should probably say uint64_t instead of uint8_t.