Doc: correct misstatement about ltree label maximum length.

The documentation says that the max length is 255 bytes, but
code inspection says it's actually 255 characters; and relevant
lengths are stored as uint16 so that that works.

doc/src/sgml/ltree.sgml

@@ -19,8 +19,8 @@
   <para>
    A <firstterm>label</firstterm> is a sequence of alphanumeric characters
    and underscores (for example, in C locale the characters
-   <literal>A-Za-z0-9_</literal> are allowed).  Labels must be less than 256 bytes
-   long.
+   <literal>A-Za-z0-9_</literal> are allowed).
+   Labels must be less than 256 characters long.
   </para>
 
   <para>