Lower the magic for the often-embed "<svg" match

So that HTML files with "<svg" in them can be recognised first and
foremost as an HTML file.

See https://bugs.webkit.org/show_bug.cgi?id=201295

freedesktop.org.xml.in

@@ -4917,6 +4917,8 @@ command to generate the output files.
     <sub-class-of type="application/xml"/>
     <magic priority="80">
       <match type="string" value="&lt;!DOCTYPE svg" offset="0:256"/>
+    </magic>
+    <magic priority="40">
       <match type="string" value="&lt;svg" offset="0:256"/>
     </magic>
     <glob pattern="*.svg"/>