fix invalid nodoc comment of Pathname#plus

Author: tompng

pathname_builtin.rb

@@ -808,7 +808,8 @@ def +(other)
   end
   alias / +
 
-  def plus(path1, path2) # -> path # :nodoc:
+  # (path1, path2) -> path
+  def plus(path1, path2) # :nodoc:
     prefix2 = path2
     index_list2 = []
     basename_list2 = []