escape dash in the middle of regexp collection ...

otherwise it makes a range, see :help /[]

Author: lyokha

syntax/pandoc.vim

@@ -445,7 +445,7 @@ if &encoding == "utf-8"
     call s:WithConceal("quotes", 'syn match pandocEndQuote /\(\>[[:punct:]]*\)\@<="[[:blank:][:punct:]\n]\@=/  containedin=pandocEmphasis,pandocStrong,pandocUListItem,pandocListItem,pandocListItemContinuation display', 'conceal cchar=”')
 endif
 " Hrule: {{{3
-syn match pandocHRule /^\s*\([*-_]\)\s*\%(\1\s*\)\{2,}$/ display
+syn match pandocHRule /^\s*\([*\-_]\)\s*\%(\1\s*\)\{2,}$/ display
 " Backslashes: {{{3
 if g:pandoc#syntax#conceal#backslash == 1
     syn match pandocBackslash /\\\@<!\\/ containedin=ALLBUT,pandocCodeblock,pandocCodeBlockInsideIndent,pandocNoFormatted,pandocNoFormattedInEmphasis,pandocNoFormattedInStrong,pandocDelimitedCodeBlock,pandocLineBlock,pandocYAMLHeader conceal