[docs] Incorrect info for multi-index upper_bound

[Avm07]

function upper_bound
const_iterator eosio::multi_index< TableName, T, Indices >::upper_bound(
    uint64_t primary
) const

Searches for the object_type with the lowest primary key that is greater than a given primary key.

Returns:
An iterator pointing to the object_type that has the highest primary key that is less than or equal to primary. If an object could not be found, it will return the end iterator. If the table does not exist** it will return -1.

Guess this is typo mistake and its return less than or equal

The -1 is wrong, iterators can't be -1 and it will return .end().