modify pattern to contain abs path in xml

- will affect only relative paths in feed.xml and featured.xml containing "../figure" which should be the case iff the path is from a knitr figure

*Note: this is grep output so "_site/featured.xml" is the filename. Characters in the text are converted for XML, but that has nothing to do with this PR. It was already happening.*

**Pre-PR XML**

_site/featured.xml:&lt;p&gt;&lt;img src=&quot;../figure/2014-10-23-SIR-and-revolution-unnamed-chunk-9-1.png&quot; title=&quot;plot of chunk unnamed-chunk-9&quot; alt=&quot;plot of chunk unnamed-chunk-9&quot; width=&quot;800px&quot; height=&quot;800px&quot; style=&quot;display: block; margin: auto;&quot; /&gt;&lt;/p&gt;

_site/feed.xml:&lt;p&gt;&lt;img src=&quot;../figure/2014-10-23-SIR-and-revolution-unnamed-chunk-9-1.png&quot; title=&quot;plot of chunk unnamed-chunk-9&quot; alt=&quot;plot of chunk unnamed-chunk-9&quot; width=&quot;800px&quot; height=&quot;800px&quot; style=&quot;display: block; margin: auto;&quot; /&gt;&lt;/p&gt;

**Post-PR XML**

_site/featured.xml:&lt;p&gt;&lt;img src=&quot;http://gallery.rcpp.org/articles/SIR-and-revolution/../figure/2014-10-23-SIR-and-revolution-unnamed-chunk-9-1.png&quot; title=&quot;plot of chunk unnamed-chunk-9&quot; alt=&quot;plot of chunk unnamed-chunk-9&quot; width=&quot;800px&quot; height=&quot;800px&quot; style=&quot;display: block; margin: auto;&quot; /&gt;&lt;/p&gt;

_site/feed.xml:&lt;p&gt;&lt;img src=&quot;http://gallery.rcpp.org/articles/SIR-and-revolution/../figure/2014-10-23-SIR-and-revolution-unnamed-chunk-9-1.png&quot; title=&quot;plot of chunk unnamed-chunk-9&quot; alt=&quot;plot of chunk unnamed-chunk-9&quot; width=&quot;800px&quot; height=&quot;800px&quot; style=&quot;display: block; margin: auto;&quot; /&gt;&lt;/p&gt;