[docs] Correct the basic syntax structure of the DISubrange example.

Notably, just close two of the debug info metadata nodes early rather
than leaving them open with `...` which won't ever lex correctly. And
add the missing `:` on the count labels.

Slowly progressing through all of the warnings on the documentation
build bot. Sorry to do this one commit at a time, but despite my best
efforts I can't trigger these errors locally.

llvm-svn: 338982