Fix accidental ABNF omission

Taking the index of the result of an index manipulation was
shown in the example and intended to work, but left out of
the ABNF apparently by accident.

Also, rework the ABNF to keep it within the text RFC line width.

Author: handrews

relative-json-pointer.xml

@@ -98,10 +98,11 @@
                 </preamble>
                 <artwork>
 <![CDATA[
-   relative-json-pointer =  non-negative-integer [index-manipulation] <json-pointer>
-   relative-json-pointer =/ non-negative-integer "#"
+   relative-json-pointer =  origin-specification <json-pointer>
+   relative-json-pointer =/ origin-specification "#"
+   origin-specification  =  non-negative-integer [index-manipulation]
    index-manipulation    =  ("+" / "-") non-negative-integer
-   non-negative-integer      =  %x30 / %x31-39 *( %x30-39 )
+   non-negative-integer  =  %x30 / %x31-39 *( %x30-39 )
            ; "0", or digits without a leading "0"
 ]]>
                 </artwork>