[formatter] change 'L'/length conversion to 'n'

docs/formatting.md

@@ -117,9 +117,9 @@ Conversion specifiers allow to *convert* the value to a different form or type.
     <td><code>["sun", "tree", "water"]</code></td>
 </tr>
 <tr>
-    <td align="center"><code>L</code></td>
+    <td align="center"><code>n</code></td>
     <td>Return the <a href="https://docs.python.org/3/library/functions.html#len" rel="nofollow">length</a> of a value</td>
-    <td><code>{foo!L}</code></td>
+    <td><code>{foo!n}</code></td>
     <td><code>7</code></td>
 </tr>
 <tr>