Update BackupAndRestore class example

[mehtamansi29]

Updated example for BackupAndRestore class example as it raises error as model is not build.
Fixed #20712

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.93%. Comparing base (5b29974) to head (5d62ed1).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20714   +/-   ##
=======================================
  Coverage   81.93%   81.93%           
=======================================
  Files         548      548           
  Lines       51190    51190           
  Branches     7912     7912           
=======================================
  Hits        41942    41942           
  Misses       7310     7310           
  Partials     1938     1938           

Flag	Coverage Δ
keras	81.76% <ø> (ø)
keras-jax	63.99% <ø> (ø)
keras-numpy	58.93% <ø> (ø)
keras-openvino	29.86% <ø> (ø)
keras-tensorflow	64.68% <ø> (ø)
keras-torch	64.06% <ø> (ø)

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