Remove spurious quote in error message. (#1202)

include/pybind11/cast.h

diff --git a/include/pybind11/cast.h b/include/pybind11/cast.h
index b3ab7f2..af6d4c7 100644
--- a/include/pybind11/cast.h
+++ b/include/pybind11/cast.h
@@ -1576,7 +1576,7 @@
         throw cast_error("Unable to cast Python instance to C++ type (compile in debug mode for details)");
 #else
         throw cast_error("Unable to cast Python instance of type " +
-            (std::string) str(handle.get_type()) + " to C++ type '" + type_id<T>() + "''");
+            (std::string) str(handle.get_type()) + " to C++ type '" + type_id<T>() + "'");
 #endif
     }
     return conv;