fix($core): cannot resolve agreement file from parent theme

Author: ulivz

packages/@vuepress/core/lib/node/App.js

@@ -385,7 +385,7 @@ module.exports = class App {
       return current
     }
     if (this.themeAPI.existsParentTheme) {
-      const parent = path.resolve(this.themeAPI.theme.path, filepath)
+      const parent = path.resolve(this.themeAPI.parentTheme.path, filepath)
       if (fs.existsSync(parent)) {
         return parent
       }

packages/@vuepress/core/lib/node/theme-api/index.js

@@ -80,7 +80,7 @@ module.exports = class ThemeAPI {
         isInternal: true
       }
       logger.warn(
-        `[vuepress] Cannot resolve Layout.vue file in \n ${Layout.path},`
+        `[vuepress] Cannot resolve Layout.vue file in \n ${Layout.path}, `
         + `fallback to default layout: ${fallbackLayoutPath}`
       )
     }