change the attribute of "notranslate" to "content" (#2050)

* change the key of "notranslate" to "content"

"value" is not a valid attribute on element meta (http://w3c.github.io/html/single-page.html#the-meta-element).
According to https://support.google.com/webmasters/answer/79812?hl=en,
content="notranslate" is recommended to use.

* don't specify the language on the Google site

src/doc/extend.md

@@ -379,7 +379,7 @@ plugin](https://www.google.com/search?ie=UTF-8&q=how+to+make+browser+search+plug
 
 * If you want to disable the translation prompt in Chrome or block Google
   Translate from translating your web page, use [`<meta name="google"
-  value="notranslate">`](https://support.google.com/translate/?hl=en#2641276).
+  content="notranslate">`](https://support.google.com/webmasters/answer/79812).
   To disable translation for a particular section of the web page, add
   [`class="notranslate"`](https://support.google.com/translate/?hl=en#2641276).