add POSIX-compliant alternative

SC3009.md

@@ -12,7 +12,19 @@ Here, `$i` expands to `{1..5}`. It does **not** expand to the sequence `1 2 3 4
 
 ### Correct code:
 
-For simple sequences of numbers, you may use the `seq` command, e.g. `seq 1 5`.
+```sh
+#!/bin/sh
+i=1
+# while $i ≤ 5 ...
+while [ "${i}" -le 5 ]; do ...
+
+  # something that will occur 5 times
+
+  i=$((i + 1))
+done
+```
+
+You can also use the `seq` command, e.g. `seq 1 5`, but `seq` is not a POSIX utility.
 
 ### Rationale: