Avoid marking fenced divs as definition lists

Workaround for vim-pandoc#236.  Make sure that the colon isn't followed by
additional colons the same way we already do for tildes.

syntax/pandoc.vim

@@ -484,7 +484,7 @@ syn match pandocListItemContinuation /^\s\+\([-+*]\s\+\|(\?.\+[).]\)\@<!\([[:upp
 
 " Definitions: {{{2
 if g:pandoc#syntax#use_definition_lists == 1
-    syn region pandocDefinitionBlock start=/^\%(\_^\s*\([`~]\)\1\{2,}\)\@!.*\n\(^\s*\n\)\=\s\{0,2}[:~]\(\~\{2,}\~*\)\@!/ skip=/\n\n\zs\s/ end=/\n\n/ contains=pandocDefinitionBlockMark,pandocDefinitionBlockTerm,pandocCodeBlockInsideIndent,pandocEmphasis,pandocStrong,pandocStrongEmphasis,pandocNoFormatted,pandocStrikeout,pandocSubscript,pandocSuperscript,pandocFootnoteID,pandocReferenceURL,pandocReferenceLabel,pandocLaTeXMathBlock,pandocLaTeXInlineMath,pandocEscapedDollar,pandocAutomaticLink,pandocEmDash,pandocEnDash,pandocFootnoteDef,pandocFootnoteBlock,pandocFootnoteID
+    syn region pandocDefinitionBlock start=/^\%(\_^\s*\([`~]\)\1\{2,}\)\@!.*\n\(^\s*\n\)\=\s\{0,2}\([:~]\)\(\3\{2,}\3*\)\@!/ skip=/\n\n\zs\s/ end=/\n\n/ contains=pandocDefinitionBlockMark,pandocDefinitionBlockTerm,pandocCodeBlockInsideIndent,pandocEmphasis,pandocStrong,pandocStrongEmphasis,pandocNoFormatted,pandocStrikeout,pandocSubscript,pandocSuperscript,pandocFootnoteID,pandocReferenceURL,pandocReferenceLabel,pandocLaTeXMathBlock,pandocLaTeXInlineMath,pandocEscapedDollar,pandocAutomaticLink,pandocEmDash,pandocEnDash,pandocFootnoteDef,pandocFootnoteBlock,pandocFootnoteID
     syn match pandocDefinitionBlockTerm /^.*\n\(^\s*\n\)\=\(\s*[:~]\)\@=/ contained contains=pandocNoFormatted,pandocEmphasis,pandocStrong,pandocLaTeXInlineMath,pandocEscapedDollar,pandocFootnoteDef,pandocFootnoteBlock,pandocFootnoteID nextgroup=pandocDefinitionBlockMark
     call s:WithConceal('definition', 'syn match pandocDefinitionBlockMark /^\s*[:~]/ contained', 'conceal cchar='.s:cchars['definition'])
 endif