Add markup to a "Python Standard Library" doc reference I added for bug 839585.

Doc/ref/ref5.tex

@@ -780,8 +780,9 @@ approximately where \code{x/y} is replaced by \code{floor(x/y)} or
 In addition to performing the modulo operation on numbers, the \code{\%}
 operator is also overloaded by string and unicode objects to perform
 string formatting (also known as interpolation). The syntax for string
-formatting is described in the Python Library Reference, section
-``Sequence Types''.
+formatting is described in the
+\citetitle[../lib/typesseq-strings.html]{Python Library Reference},
+section ``Sequence Types''.
 
 \deprecated{2.3}{The floor division operator, the modulo operator,
 and the \function{divmod()} function are no longer defined for complex