Documentation for BlockSparseArray incorrectly states access takes linear time

[nisamson]

https://github.com/dotnetrdf/vds-common/blob/aeaee13281767f5e97946ffe360663ba99c8f937/src/Collections/BlockSparseArray.cs#L37C143-L37C143

The documentation here suggests accessing elements in a BlockSparseArray takes linear time, when the wiki documentation states it is a constant-time access (which appears to be correct, since accessing an element requires a total of two constant-time array indexes).