Fix ShouldBindBodyWith docs example using binding.Form

[Hashim1999164]

Summary

• The ShouldBindBodyWith example in docs/doc.md used binding.Form, which does not implement BindingBody, so that call is invalid.
• Updated the example to use binding.JSON, which matches the supported body binders listed in the same section.

Fixes #3296

Test plan

• Confirm the example in docs/doc.md compiles conceptually against ShouldBindBodyWith / BindingBody
• Skim the surrounding docs note that Form should use ShouldBind instead of ShouldBindBodyWith

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.32%. Comparing base (3dc1cd6) to head (909b6f3).
⚠️ Report is 289 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4766      +/-   ##
==========================================
- Coverage   99.21%   98.32%   -0.89%     
==========================================
  Files          42       48       +6     
  Lines        3182     3172      -10     
==========================================
- Hits         3157     3119      -38     
- Misses         17       43      +26     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.31% <ø> (?)
-tags go_json	98.25% <ø> (?)
-tags nomsgpack	98.30% <ø> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.32% <ø> (?)
go-1.26	98.32% <ø> (?)
macos-latest	98.32% <ø> (-0.89%)	⬇️
ubuntu-latest	98.32% <ø> (-0.89%)	⬇️

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