doc/doxygen/chapters/07data_management.doxy: fix formatting

doc/doxygen/chapters/07data_management.doxy

@@ -1,7 +1,7 @@
 /*
  * This file is part of the StarPU Handbook.
  * Copyright (C) 2009--2011  Universit@'e de Bordeaux
- * Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015  CNRS
+ * Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015, 2016  CNRS
  * Copyright (C) 2011, 2012 INRIA
  * See the file version.doxy for copying conditions.
  */
@@ -230,7 +230,9 @@ And now we can submit tasks working on vert_handle handles (and not on the main
 handle any more). Eventually we want to work on the main handle again, so we
 submit the unpartitioning:
 
+\code{.c}
 starpu_data_unpartition_submit(handle, PARTS, vert_handle, -1);
+\endcode
 
 And now we can submit tasks working on the main handle again.