[clangd] Fix markdown rendering in VSCode
Eventough it is OK to have a new line without any preceding spaces in
some markdown specifications, VSCode requires two spaces before a new line to
break a line inside a paragraph.
Reviewers: sammccall, ilya-biryukov
Subscribers: MaskRay, jkorous, arphaman, usaxena95, cfe-commits
Differential Revision: https://reviews.llvm.org/D72462