link to correct migration guide

Author: vkarpov15

docs/layout.pug

@@ -139,7 +139,7 @@ html(lang='en')
                   li.pure-menu-item.sub-item
                     a.pure-menu-link(href=`${versions.versionedPath}/docs/api/virtualtype.html`, class=outputUrl === `${versions.versionedPath}/docs/api/virtualtype.html` ? 'selected' : '') VirtualType
               li.pure-menu-item
-                a.pure-menu-link(href=`${versions.versionedPath}/docs/migrating_to_8.html`, class=outputUrl === `${versions.versionedPath}/docs/migrating_to_8.html` ? 'selected' : '') Migration Guide
+                a.pure-menu-link(href=`${versions.versionedPath}/docs/migrating_to_9.html`, class=outputUrl === `${versions.versionedPath}/docs/migrating_to_9.html` ? 'selected' : '') Migration Guide
               li.pure-menu-item
                 a.pure-menu-link(href=`${versions.versionedPath}/docs/compatibility.html`, class=outputUrl === `${versions.versionedPath}/docs/compatibility.html` ? 'selected' : '') Version Compatibility
               li.pure-menu-item
@@ -176,4 +176,3 @@ html(lang='en')
         script(type="text/javascript" src=`${versions.versionedPath}/docs/js/mobile-navbar-toggle.js`)
 
         script(type="text/javascript" src=`${versions.versionedPath}/docs/js/copy-code.js`)
-