Update doc/specs/stdlib_math.md

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

Author: ejovo13

doc/specs/stdlib_math.md

@@ -113,7 +113,7 @@ Function.
 
 `start`: Shall be scalar of any numeric type. This argument is `intent(in)`.
 `end`: Shall be the same `type` and `kind` as `start`. This argument is `intent(in)`.
-`n`: Shall be an integer specifying the length of the output. This argument is `intent(in)`.
+`n`: Shall be an integer specifying the length of the output. This argument is `optional` and `intent(in)`.
 
 #### Output value or Result value