[llvm] [docs][PDB] Fix MSF Free Block Map interval wording (PR #214677)

[Hans Wennborg via llvm-commits]

https://github.com/zmodem commented:

Thanks, this is a good improvement. (Though I still find the "intervals" terminology somewhat confusing in this doc.)

The PR needs rebasing on top of d0b078e (the document was converted to markdown).

https://github.com/llvm/llvm-project/pull/214677