Revert "chore: Remove unnecessary argument for join method"

This reverts commit 4a1c74b.

Since RDoc still supports ruby 2.6 which has not deprecated `$,`, the
argument of `Array#join` is not unnecessary yet.

Author: nobu

lib/rdoc/token_stream.rb

@@ -112,7 +112,7 @@ def token_stream
   # Returns a string representation of the token stream
 
   def tokens_to_s
-    token_stream.compact.map { |token| token[:text] }.join
+    token_stream.compact.map { |token| token[:text] }.join ''
   end
 
 end