Fix git clang-format help message

[albestro]

The helper message reports

clangFormat.extension (singular)

but in the code clangformat.extensions (plural) is used.

llvm-project/clang/tools/clang-format/git-clang-format

Lines 46 to 49 in fddacd0

	clangFormat.binary
	clangFormat.commit
	clangFormat.extension
	clangFormat.style

For what concerns the case-sensitiveness, it seems that options names are case-insensitive.

[tclin914]

Fixed by https://reviews.llvm.org/rGd40afadec0ac.