pattern, regexp: Declare the variable in the documentation example

Fixes #590.

docs/highlighters/regexp.md

@@ -10,6 +10,7 @@ patterns.
 To use this highlighter, associate regular expressions with styles in the
 `ZSH_HIGHLIGHT_REGEXP` associative array, for example in `~/.zshrc`:
 
+    typeset -A ZSH_HIGHLIGHT_PATTERNS
     ZSH_HIGHLIGHT_REGEXP+=('\bsudo\b' fg=123,bold)
 
 This will highlight "sudo" only as a complete word, i.e., "sudo cmd", but not