Update default fortran example (#4807)

integer reflects a more accurate example compared to other languages in
the code

examples/fortran/default.f90

@@ -1,7 +1,6 @@
 ! Type your code here, or load an example.
-real function square(x)
+integer function square(x)
     implicit none
-    real, intent(in) :: x
+    integer, intent(in) :: x
     square = x * x
-    return
 end function square