Update src/stdlib_ascii.f90

Co-authored-by: Ian Giestas Pauli <iangiestaspauli@gmail.com>

Author: arjenmarkus

src/stdlib_ascii.f90

@@ -144,7 +144,8 @@ pure logical function is_graphical(c)
     pure logical function is_printable(c)
         character(len=1), intent(in) :: c !! The character to test.
         integer :: ic
-        ic = iachar(c)                    ! '~'
+        ic = iachar(c)
+        !The character is printable if it's between ' ' and '~' in the ASCII table
         is_printable = ic >= iachar(' ') .and. ic <= int(z'7E')
     end function