doc: Fix typo.

doc/chapters/basic-api.texi

@@ -2548,7 +2548,7 @@ collect statistics about the kernel execution (used cycles, consumed power).
 @node OpenCL utilities
 @subsection OpenCL utilities
 
-@deftypefun {const char *}starpu_opencl_error_string (cl_int @var{status})
+@deftypefun {const char *} starpu_opencl_error_string (cl_int @var{status})
 Return the error message in English corresponding to @var{status}, an
 OpenCL error code.
 @end deftypefun