Fix SubEntry code examples.

[redboltz]

Update the version to 0.7.3.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.91%. Comparing base (199dca1) to head (7ba8636).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #56   +/-   ##
=======================================
  Coverage   90.91%   90.91%           
=======================================
  Files          55       55           
  Lines        7470     7470           
=======================================
  Hits         6791     6791           
  Misses        679      679           

Files with missing lines	Coverage Δ
src/mqtt/packet/v3_1_1/subscribe.rs	87.36% <ø> (ø)
src/mqtt/packet/v5_0/subscribe.rs	89.25% <ø> (ø)

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