Highlight all new lines in docs example upgrade

[cdeil]

No description provided.

[codecov]

Codecov Report

Merging #795 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #795   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         286    288    +2     
  Lines        7466   7555   +89     
=====================================
+ Hits         7466   7555   +89

Impacted Files	Coverage Δ
docs/src/openapi_callbacks/tutorial001.py	100% <0%> (ø)	⬆️
fastapi/encoders.py	100% <0%> (ø)	⬆️
fastapi/routing.py	100% <0%> (ø)	⬆️
tests/test_inherited_custom_class.py	100% <0%> (ø)
tests/test_sub_callbacks.py	100% <0%> (ø)

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[tiangolo]

Thanks @cdeil ! 📝

My original intention was to highlight just the input body, but I see how this can look strange, as the rest of the new code is highlighted, so, let's update it ✔️ 🎉