fix: Update json package README.md

[MalekLahbib]

in the three examples provided in the README.md file, there's a useless import of "fmt" package.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.96%. Comparing base (628f965) to head (5ce4f7a).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2921      +/-   ##
==========================================
- Coverage   60.96%   60.96%   -0.01%     
==========================================
  Files         564      564              
  Lines       75273    75273              
==========================================
- Hits        45893    45889       -4     
- Misses      26009    26016       +7     
+ Partials     3371     3368       -3     

Flag	Coverage Δ
contribs/gnodev	61.46% <ø> (ø)
contribs/gnofaucet	14.46% <ø> (ø)
gno.land	67.92% <ø> (ø)
gnovm	65.78% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	20.23% <ø> (+0.35%)	⬆️
tm2	62.04% <ø> (-0.09%)	⬇️

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