docs: removes unimplemented attributes in the extproc example

[mathetake]

What type of PR is this?
the title says.

What this PR does / why we need it:
This document is misleading by using unimplemented field named "attributes"
which presumably will be implemented in #3170

Which issue(s) this PR fixes:
N/A

[arkodg]

thanks !

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.91%. Comparing base (cc8acbd) to head (a418e0d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4115      +/-   ##
==========================================
+ Coverage   67.90%   67.91%   +0.01%     
==========================================
  Files         187      187              
  Lines       23032    23032              
==========================================
+ Hits        15639    15643       +4     
+ Misses       6273     6271       -2     
+ Partials     1120     1118       -2     

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