driver: Followup to 80148f6c840299f0980f4359ec5307ca63837dff: don't squat on the highlighters' namespace.

zsh-syntax-highlighting.zsh

@@ -88,7 +88,7 @@ _zsh_highlight()
     local highlighter; for highlighter in $ZSH_HIGHLIGHT_HIGHLIGHTERS; do
 
       # eval cache place for current highlighter and prepare it
-      cache_place="_zsh_highlight_highlighter_${highlighter}_cache"
+      cache_place="_zsh_highlight__highlighter_${highlighter}_cache"
       typeset -ga ${cache_place}
 
       # If highlighter needs to be invoked