pattern, regexp: Declare the variable in the documentation example

Fixes #590.

docs/highlighters/pattern.md

@@ -9,6 +9,9 @@ This is the `pattern` highlighter, that highlights user-defined patterns.
 To use this highlighter, associate patterns with styles in the
 `ZSH_HIGHLIGHT_PATTERNS` associative array, for example in `~/.zshrc`:
 
+    # Declare the variable
+    typeset -A ZSH_HIGHLIGHT_PATTERNS
+
     # To have commands starting with `rm -rf` in red:
     ZSH_HIGHLIGHT_PATTERNS+=('rm -rf *' 'fg=white,bold,bg=red')