[HOT FIX] Fix broken display_formula.png by using relative path

[AhyoungRyu]

What is this PR for?

Currently display_formula.png (added by #1606) is broken in here: http://zeppelin.apache.org/docs/0.7.0-SNAPSHOT/displaysystem/basicdisplaysystem.html#mathematical-expressions

Fixed it by using relative path for the image. But have no idea why the other image files are not broken in that page.

What type of PR is it?

Documentation | Hot Fix

Todos

• - Task

What is the Jira issue?

N/A

How should this be tested?

It can't be reproduced using docs dev mode. Needs to be tested with below steps.

1) build gh-pages (website) branch
JEKYLL_ENV=production bundle exec jekyll build
cp -r _site/ /tmp/zeppelin_website/
mkdir -p /tmp/zeppelin_website/docs/0.7.0-SNAPSHOT

2) build this patch (docs) and copy it under docs/0.7.0-SNAPSHOT of website
cd docs
bundle exec jekyll build --safe
cp -r _site/ /tmp/zeppelin_website/0.7.0-SNAPSHOT/

3) start httpserver and browse http://localhost:8000/docs/0.7.0-SNAPSHOT/
cd /tmp/zeppelin_website
python -m SimpleHTTPServer

Screenshots (if appropriate)

Before

• In the official website

GET http://zeppelin.apache.org/assets/themes/zeppelin/img/screenshots/display_formula.png 404 (Not Found)

After

• test locally using same with production path

Questions:

• Does the licenses files need update?
• Is there breaking changes for older versions?
• Does this needs documentation?

[astroshim]

LGTM!!

[Leemoonsoo]

LGTM.
Merge it as a hotfix.