[clangd] Fix markdown rendering in VSCode

Summary:
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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72462