[libsigc++2/variadic_bind4] Fix a typo in a static_asser() error message.

[Murray Cumming <murrayc src gnome org>]

commit 9dd5fdcbeedddd62af0f61442be4ac92b9205947
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Mar 3 23:00:31 2016 +0100

    Fix a typo in a static_asser() error message.

 sigc++/adaptors/bind.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/adaptors/bind.h b/sigc++/adaptors/bind.h
index 2980202..3046c04 100644
--- a/sigc++/adaptors/bind.h
+++ b/sigc++/adaptors/bind.h
@@ -130,7 +130,7 @@ struct bind_functor : public adapts<T_functor>
       
       //Prevent calling tuple_start<> with values that will cause a compilation error.
       static_assert(I_location <= t_args_size,
-        "I_location must be less or equal to the number of arguments.");
+        "I_location must be less than or equal to the number of arguments.");
 
       auto t_start = internal::tuple_start<I_location>(t_args);
       auto t_bound = internal::tuple_transform_each<internal::TransformEachInvoker>(bound_);