Fix highlighting of tags

in some circumstances, e.g.

- :Tag:

the tag wasn't highlighted. Don't know why.

syntax/vimwiki_default.vim

@@ -92,5 +92,5 @@ let g:vimwiki_rxMathStart = '{{\$'
 let g:vimwiki_rxMathEnd = '}}\$'
 
 let g:vimwiki_rxComment = '^\s*%%.*$'
-let g:vimwiki_rxTags = '\(^\|\s\)\zs:\([^:[:space:]]\+:\)\+\ze\(\s\|$\)'
+let g:vimwiki_rxTags = '\%(^\|\s\)\@<=:\%([^:[:space:]]\+:\)\+\%(\s\|$\)\@='
 " see also g:vimwiki_default_tag_search