Updated SC3009 (markdown)

SC3009.md

@@ -1,9 +1,21 @@
-**In POSIX `sh`, brace expansion is undefined.**
+## In POSIX `sh`, brace expansion is undefined.
+
+### Problematic code:
 
-Problematic code:
 ```
 #!/bin/sh
 
 for i in {1..5}; do ...
 ```
+
 Here, `$i` expands to `{1..5}`. It does **not** expand to the sequence `1 2 3 4 5`
+
+### Correct code:
+
+For simple sequences of numbers, you may use the `seq` command, e.g. `seq 1 5`.
+
+### Rationale:
+
+### Exceptions:
+
+None