docs(img-tag): title text should be quoted

[curbengh]

Check List

Please read and check followings before submitting a PR.

• Others (Update, fix, translation, etc...)

The current example:

{% img class-name /path/to/image 100 100 "title text 'alt text'" %}

<img src="/path/to/image" class="class-name" width="100" height="100" title="title text " alt="alt text">

Notice the trailing space in the title attribute.

The correct usage is to quote both title and alt, then quote both of them:

{% img class-name /path/to/image 100 100 '"title text" "alt text"' %}

<img src="/path/to/image" class="class-name" width="100" height="100" title="title text" alt="alt text">

"'title text' 'alt text'" also works.

Closes hexojs/hexo#1277