GUACAMOLE-773: Remove use of XHTML-style self-closing tags from normal HTML.

This is particularly important for AngularJS 1.8.x and jQuery 3.5.0+,
which will no longer correct this automatically with their HTML
prefilters, resulting in unexpected nesting of tags when the browser
interprets something like "<div/>" as "<div>" (older versions of jQuery
would have corrected "<div/>" to "<div></div>").

See:

 * https://docs.angularjs.org/guide/migration#migrating-from-1-7-to-1-8
 * https://jquery.com/upgrade-guide/3.5/

doc/guacamole-example/src/main/webapp/index.html

@@ -20,7 +20,7 @@
 <html>
 
     <head>
-        <link rel="stylesheet" type="text/css" href="guacamole.css"/>
+        <link rel="stylesheet" type="text/css" href="guacamole.css">
         <title>Guacamole (EXAMPLE)</title>
     </head>