XWIKI-19292: Fix bad escaping

(cherry picked from commit 27f83913)

xwiki-platform-core/xwiki-platform-wiki/xwiki-platform-wiki-ui/xwiki-platform-wiki-ui-mainwiki/src/main/resources/WikiManager/JoinWiki.xml

@@ -245,7 +245,7 @@
                   <a href="$backUrl" class='button secondary'>{{translation key="platform.wiki.users.join.request.cancel.label"/}}</a>
                 </span>
                 <input type='hidden' name='wikiId' value="$!wikiId"/>
-                <input type='hidden' name='requestJoin' value="$!request.requestJoin"/>
+                <input type='hidden' name='requestJoin' value="$escapetool.xml($!request.requestJoin)"/>
                 <input type="hidden" name="form_token" value="$!escapetool.xml($services.csrf.getToken())" />
               </dl>
             </form>