Fix the search error when use Latex

[fooage]

I fixed a bug about simple-jekyll-search's config. It will occurred an error when use special characters like $ and \ etc.

• error: SimpleJekyllSearch --- failed to get JSON (/search.json)

This is because special characters can cause regular expression errors and thus fail to generate search.json.

I had find the solution in https://github.com/christian-fei/Simple-Jekyll-Search, and it now works great with Latex formulas and special characters. Because it will escape special characters. 😊

Only changed { post.content | strip_html | strip_newlines | truncate:400 | jsonify } and removed extra quotation marks~