doc/chapters/c-extensions.texi: correct link name, the html file is called starpu.html not gcc.html

doc/chapters/c-extensions.texi

@@ -75,7 +75,7 @@ choose any of its implementations.
 
 Tasks and their implementations must be @emph{declared}.  These
 declarations are annotated with @dfn{attributes} (@pxref{Attribute
-Syntax, attributes in GNU C,, gcc, Using the GNU Compiler Collection
+Syntax, attributes in GNU C,, starpu, Using the GNU Compiler Collection
 (GCC)}): the declaration of a task is a regular C function declaration
 with an additional @code{task} attribute, and task implementations are
 declared with a @code{task_implementation} attribute.