chore(docs): tutorial typo nullable schema defined

[redrebel]

I am getting start with tutorial.

and I found something wrong typo.

t.nonNull.field("createDraft", {...})

then, generated SDL as

createDraft(body: String!, title: String!): Post!

and

t.nonNull.list.field("drafts", {...}) => drafts: [Post]!
...
t.list.field("posts", { ...}}  => posts: [Post]

[codecov]

Codecov Report

Merging #675 (f21a8cf) into develop (24a78e8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #675   +/-   ##
========================================
  Coverage    93.61%   93.61%           
========================================
  Files           44       44           
  Lines         2865     2865           
  Branches       668      668           
========================================
  Hits          2682     2682           
  Misses         181      181           
  Partials         2        2           

[jasonkuhrt]

ty!