Update doc/specs/stdlib_math.md

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

Author: jalvesz

doc/specs/stdlib_math.md

@@ -62,7 +62,7 @@ Here inputs are of type `real` and kind `sp`
 ```
 
 <!-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -->
-### `swap` function
+### `swap` subroutine
 
 #### Description