add S on ch06

ch06_insert_mode.md

@@ -15,6 +15,7 @@ A    Append text at the end of line
 o    Starts a new line below the cursor and insert text
 O    Starts a new line above the cursor and insert text
 s    Delete the character under the cursor and insert text
+S    Delete the current line and insert text
 gi   Insert text in same position where the last insert mode was stopped in current buffer
 gI   Insert text at the start of line (column 1)
 ```