Merge pull request #139 from CyrusYip/patch-5

`S` is synonym for `cc`

ch06_insert_mode.md

@@ -16,7 +16,7 @@ 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
+S    Delete the current line and insert text, synonym for "cc"
 gi   Insert text in same position where the last insert mode was stopped
 gI   Insert text at the start of line (column 1)
 ```