Update overview.rst | Fix an inconsistency

[wangqin0]

There exists an inconsistency between the code (line 37 - 38) and the output 'quotes.json' (line 56 - 68).

Note that even though according to line 53 - 54 'quotes.json' is "reformatted here for better readability", it cannot explain why the "author" field precedes the "text" field.

Intended output for the code BEFORE change:
[{
"text": "\u201cThe person, be it gentleman or lady, who has not pleasure in a good novel, must be intolerably stupid.\u201d",
"author": "Jane Austen"
},
{
"text": "\u201cOutside of a dog, a book is man's best friend. Inside of a dog it's too dark to read.\u201d",
"author": "Groucho Marx"
},
{
"text": "\u201cA day without sunshine is like, you know, night.\u201d",
"author": "Steve Martin"
},
...]

Intended output for the code After change (the inconsistency is fixed):
[{
"author": "Jane Austen",
"text": "\u201cThe person, be it gentleman or lady, who has not pleasure in a good novel, must be intolerably stupid.\u201d"
},
{
"author": "Groucho Marx",
"text": "\u201cOutside of a dog, a book is man's best friend. Inside of a dog it's too dark to read.\u201d"
},
{
"author": "Steve Martin",
"text": "\u201cA day without sunshine is like, you know, night.\u201d"
},
...]

[codecov]

Codecov Report

Merging #4213 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4213      +/-   ##
==========================================
- Coverage   83.74%   83.73%   -0.02%     
==========================================
  Files         165      165              
  Lines        9713     9713              
  Branches     1446     1446              
==========================================
- Hits         8134     8133       -1     
  Misses       1332     1332              
- Partials      247      248       +1

Impacted Files	Coverage Δ
scrapy/utils/trackref.py	83.78% <0%> (-2.71%)	⬇️

[Gallaecio]

CPython 3.5, which Scrapy will probably support until it reaches end-of-life (September) or a bit longer, does not keep insert order in dictionaries. So the output was probably copy-pasted from CPython 3.5 or earlier.

+1 to the change, though. Consistency is key!