Fix a type error in rst' sub list. (#2881)
Fix a type error in rst' sub list. (#2881)

Sub list in rst requires a blank line above and below.

Ref:
- http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#bullet-lists
asdf2014/superset,zhouyao1994/incubator-superset,timifasubaa/incubator-superset,BRD-CD/superset,BRD-CD/superset,massgov/incubator-superset,nekia/incubator-superset-dev,dmigo/incubator-superset,SingTel-DataCo/incubator-superset,airbnb/caravel,airbnb/superset,airbnb/superset,asdf2014/superset,apache/incubator-superset,alanmcruickshank/superset-dev,timifasubaa/incubator-superset,mistercrunch/panoramix,zhouyao1994/incubator-superset,alanmcruickshank/superset-dev,apache/incubator-superset,FrederichCheng/incubator-superset,SingTel-DataCo/incubator-superset,nekia/incubator-superset-dev,FrederichCheng/incubator-superset,airbnb/superset,nekia/incubator-superset-dev,massgov/incubator-superset,dmigo/incubator-superset,dmigo/incubator-superset,dmigo/incubator-superset,airbnb/caravel,zhouyao1994/incubator-superset,airbnb/caravel,SingTel-DataCo/incubator-superset,FrederichCheng/incubator-superset,apache/incubator-superset,airbnb/caravel,BRD-CD/superset,timifasubaa/incubator-superset,alanmcruickshank/superset-dev,BRD-CD/superset,mistercrunch/panoramix,alanmcruickshank/superset-dev,nekia/incubator-superset-dev,apache/incubator-superset,asdf2014/superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,timifasubaa/incubator-superset,massgov/incubator-superset,massgov/incubator-superset,FrederichCheng/incubator-superset,apache/incubator-superset,SingTel-DataCo/incubator-superset,mistercrunch/panoramix,mistercrunch/panoramix,airbnb/superset,asdf2014/superset