Fix xhtml namespace typo (#4131)

src/platforms/web/util/element.js

@@ -6,7 +6,7 @@ import { makeMap } from 'shared/util'
 export const namespaceMap = {
   svg: 'http://www.w3.org/2000/svg',
   math: 'http://www.w3.org/1998/Math/MathML',
-  xhtml: 'http://www.w3.org/1999/xhtm'
+  xhtml: 'http://www.w3.org/1999/xhtml'
 }
 
 export const isHTMLTag = makeMap(