Fix escaping on builds' REST API page

The "Accessing Progressive Console Output" section of builds' REST API
page is displayed incorrectly.  It should contain "<pre>", but the
"&lt;" is replaced by a less-than sign.  This causes a <pre> tag to be
applied to the rest of the section.  Change the & to &amp; to prevent
this.

core/src/main/resources/hudson/model/Run/_api.jelly

@@ -46,7 +46,7 @@ THE SOFTWARE.
     You can retrieve in-progress console output by making repeated GET requests with a parameter.
     You'll basically send GET request to
     <a href="../logText/progressiveText?start=0">this URL</a> (or <a href="../logText/progressiveHtml?start=0">this URL</a> if you
-    want HTML that can be put into &lt;pre> tag.)
+    want HTML that can be put into &amp;lt;pre> tag.)
     The <tt>start</tt> parameter controls the byte offset of where you start.
   </p><p>
     The response will contain a chunk of the console output, as well as the <tt>X-Text-Size</tt> header that represents