Fix typo

Author: jjbohn

object.textile

@@ -398,7 +398,7 @@ However, in C, `nil` (`Qnil`) is true. That's why in C a Ruby-style macro,
 As in `Qnil` only the third lower bit is 1, in `~Qnil` only the third lower
 bit is 0. Then only `Qfalse` and `Qnil` become 0 with a bitwise and.
 
-`!=0` has be added to be certain to only have 0 or 1, to satisfy the
+`!=0` has been added to be certain to only have 0 or 1, to satisfy the
 requirements of the glib library that only wants 0 or 1
 ("[ruby-dev:11049]":http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-dev/11049).