issue-665: fix example when using json=",string"

[tcarreira]

Describe the PR
Fix the json example when a struct field has the json:",string" tag.

Relation issue
#665

Additional context
encoding/json states:

The "string" option signals that a field is stored as JSON inside a JSON-encoded string. It applies only to fields of string, floating point, integer, or boolean types. This extra level of encoding is sometimes used when communicating with JavaScript programs:

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Before this fix, the provided example would fail json.Unmarshal:

After fix:

[codecov-io]

Codecov Report

Merging #677 into master will increase coverage by 0.29%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #677      +/-   ##
==========================================
+ Coverage   85.00%   85.29%   +0.29%     
==========================================
  Files           7        7              
  Lines        1807     1823      +16     
==========================================
+ Hits         1536     1555      +19     
+ Misses        172      171       -1     
+ Partials       99       97       -2     

Impacted Files	Coverage Δ
parser.go	80.42% <90.00%> (+0.63%)	⬆️

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[sdghchj]

structField.schemaType = "string"

[sdghchj]

LGTM