Use std::lower_bound in notes for std::partition_point

[mishasweetpie]

Changed upper_bound to lower_bound since lower_bound is actually similar to std::partition_point rather than upper_bound.

From notes about std::partition point on cppreference.com

This algorithm is a more general form of std::lower_bound, which can be expressed in terms of std::partition_point with the predicate [&](auto const& e) { return e < value; });