[clang-format] [NFC] Fix spelling and grammatical errors in IncludeBlocks text

Fix spelling mistake
Leave space after `.` and before beginning of next sentence
Reword it slightly to try and make it more readable.
Ensure RST is updated correctly (it generated a change)

Reviewed By: HazardyKnusperkeks, curdeius

Differential Revision: https://reviews.llvm.org/D92822