Fix box_index_count return value description

[mandesero]

Info

Root document: https://www.tarantool.io/en/doc/latest/reference/reference_capi/box_index/#c.box_index_count

What's wrong?

The description of the return value is inaccurate:

-: return: 0 on success
+: return: >= 0 otherwise

The function returns the number of matched tuples (which can be any non-negative value), not just 0.