Update doc/specs/stdlib_linalg_iterative_solvers.md

Co-authored-by: Jeremie Vandenplas <jeremie.vandenplas@gmail.com>

Author: jalvesz

doc/specs/stdlib_linalg_iterative_solvers.md

@@ -21,7 +21,7 @@ The `linop_<kind>_type` derive type is an auxiliary class enabling to abstract t
 
 #### Type-bound procedures
 
-The following type-bound procedures pointer enable customization of the solver:
+The following type-bound procedure pointers enable customization of the solver:
 
 ##### `matvec`