[MINOR][DOC] improve the doc for "spark.memory.offHeap.size"

The description of "spark.memory.offHeap.size" in the current document does not clearly state that memory is counted with bytes....

This PR contains a small fix for this tiny issue

document fix

Author: CodingCat <zhunansjtu@gmail.com>

Closes #11561 from CodingCat/master.