message: fixing doctest
diff: @@ -158,7 +158,7 @@ impl<'b> TokenStream for Box<TokenStream + 'b> {
  /// use tantivy::tokenizer::*;
  ///
  /// # fn main() {
  -/// let mut tokenizer = SimpleTokenizer
  +/// let tokenizer = SimpleTokenizer
  /// .filter(RemoveLongFilter::limit(40))
  /// .filter(LowerCaser);
  /// let mut token_stream = tokenizer.token_stream("Hello, happy tax payer");
  