Add space before closing slash in empty elements

This change is not necessary for HTML5 output, as the toolkit automatically generates the proper HTML5 syntax for void elements per https://dev.w3.org/html5/html-author/#void.

However, to ensure these examples also work in XHTML customizations, this commit adds the space before the slash in empty elements per compatibility guidelines in XHTML 1.0, Appendix C: https://www.w3.org/TR/xhtml-media-types/#C_2

Signed-off-by: Roger Sheen <roger@infotexture.net>

resources/header.xml

@@ -1,4 +1,4 @@
 <div class="header">
   <p>DITA Open Toolkit</p>
-  <hr/>
+  <hr />
 </div>

samples/plugins/com.example.html5-webfont/include/webfont.hdf.xml

@@ -1,3 +1,3 @@
 <div>
-  <link href="https://fonts.googleapis.com/css?family=Noto+Sans" rel="stylesheet"></link>
+  <link href="https://fonts.googleapis.com/css?family=Noto+Sans" rel="stylesheet" />
 </div>