Use valid css class names when pygments language contains invalid chars, eg html+django

Pygments has some lexers that combine multiple languages and are referenced by name using the plus sign, eg "html+django" is a valid lexer name.

In the ``highlight`` tag, the lexer name also gets used as the CSS class name in the wrapping HTML <code> block, but the plus sign is invalid here.

This change simply converts plus signs to minus signs, to conform with valid CSS class names.