Update doc/specs/stdlib_linalg_iterative_solvers.md

Co-authored-by: Jeremie Vandenplas <[email protected]>

Author: jalvesz

doc/specs/stdlib_linalg_iterative_solvers.md

@@ -240,7 +240,7 @@ Subroutine
 
 `precond` (optional): scalar of type `integer` enabling to switch among the default preconditioners available. If no value is given, no preconditionning will be applied. This argument is `intent(in)`.
 
-`M` (optional): `class(linop_<kind>_type)` defining a custom preconditioner linear operator. If given, `precond` will have no effect, a pointer is set to this custom preconditioner.
+`M` (optional): scalar derived type of `class(linop_<kind>_type)` defining a custom preconditioner linear operator. If given, `precond` will have no effect, a pointer is set to this custom preconditioner.
 
 `workspace` (optional): `type(solver_workspace_<kind>_type)` holding the work temporal array for the solver. If the user passes its own `workspace`, then internally a pointer is set to it, otherwise, memory will be internally allocated and deallocated before exiting the procedure. This argument is `intent(inout)`.