[xml/sgml-pkgs] Bug#749092: add --xpath example

[積丹尼 Dan Jacobson]

Package: libxml2-utils
Version: 2.9.1+dfsg1-3
Severity: wishlist
File: /usr/share/man/man1/xmllint.1.gz

At
       --xpath "XPath_expression"
           Run an XPath expression given as argument and print the result. In
           case of a nodeset result, each node in the node set is serialized
           in full in the output. In case of an empty node set the "XPath set
           is empty" result will be shown and an error exit code will be
           returned.

add an example. Else one does not know how to use this offline just
reading the man page.

OK I figured out one way,
xmllint --xpath '//text()' --html file.html