Tweaked walkthrough readability
Tweaked walkthrough readability

- Fixed some command examples, e.g., no """ string block literals in
  psql command examples. (It's not a valid shell syntax.)
- Polished SQL formatting.
- Wrapped `extraction.extractors { }` around extractors for readability.

HazyResearch/deepdive,vasyvas/deepdive,AlexisBRENON/deepdive,kod3r/deepdive,wgapl/deepdive,sky-xu/deepdive,maalmeida1837/deepdive,zifeishan/deepdive,RTsWorld/deepdive,maalmeida1837/deepdive,topojoy/deepdive,closedLoop/deepdive,nvoron23/deepdive,sai16vicky/deepdive,AlexisBRENON/deepdive,vsoch/deepdive,HazyResearch/deepdive,AlexisBRENON/deepdive,Atlas7/deepdive,nvoron23/deepdive,maalmeida1837/deepdive,nvoron23/deepdive,Atlas7/deepdive,infinitespace/deepdive,Gela/deepdive,kod3r/deepdive,sai16vicky/deepdive,gaapt/deepdive,topojoy/deepdive,feiranwang/deepdive,feiranwang/deepdive,gaapt/deepdive,infinitespace/deepdive,sky-xu/deepdive,gaapt/deepdive,zifeishan/deepdive,infinitespace/deepdive,kod3r/deepdive,vsoch/deepdive,vasyvas/deepdive,closedLoop/deepdive,AlexisBRENON/deepdive,zifeishan/deepdive,wgapl/deepdive,Atlas7/deepdive,nvoron23/deepdive,gaapt/deepdive,HazyResearch/deepdive,vsoch/deepdive,gaapt/deepdive,infinitespace/deepdive,shahin/deepdive,maalmeida1837/deepdive,Gela/deepdive,sai16vicky/deepdive,RTsWorld/deepdive,topojoy/deepdive,RTsWorld/deepdive,HazyResearch/deepdive,gaapt/deepdive,nvoron23/deepdive,wgapl/deepdive,closedLoop/deepdive,topojoy/deepdive,vasyvas/deepdive,feiranwang/deepdive,Gela/deepdive,RTsWorld/deepdive,vsoch/deepdive,maalmeida1837/deepdive,wgapl/deepdive,vsoch/deepdive,zifeishan/deepdive,kod3r/deepdive,Gela/deepdive,Gela/deepdive,topojoy/deepdive,infinitespace/deepdive,closedLoop/deepdive,kod3r/deepdive,AlexisBRENON/deepdive,vasyvas/deepdive,shahin/deepdive,RTsWorld/deepdive,closedLoop/deepdive,Atlas7/deepdive,feiranwang/deepdive,sai16vicky/deepdive,wgapl/deepdive,infinitespace/deepdive,sai16vicky/deepdive,zifeishan/deepdive,Atlas7/deepdive,vasyvas/deepdive