fix API documentation for readProperty

[BinkertR]

fix the API description of how to read a property value.
As described in #1344

[danielpeintner]

Thanks @BinkertR for your work 👍

I would like to wait a bit since there are more things that are no longer correct and I suggest to clarify first what to do with this MD document as mentioned here.

Your opinion is welcome too!

[relu91]

I would accept this small fix even if most of the content needs to be moved and fixed it is still correct and valuable.

[danielpeintner]

It is fine by me also even if more work is needed.

@BinkertR can you look into the Eclipse ECA issue.. you need to register and accept the Eclipse rules once
At the moment, your GitHub user isn't known to Eclipse
https://api.eclipse.org/git/eca/status/gh/eclipse-thingweb/node-wot/1345

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.59%. Comparing base (2b1e766) to head (4ef0143).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1345      +/-   ##
==========================================
- Coverage   78.58%   77.59%   -0.99%     
==========================================
  Files          83       83              
  Lines       16588    16588              
  Branches     1612     1606       -6     
==========================================
- Hits        13035    12872     -163     
- Misses       3529     3686     +157     
- Partials       24       30       +6     

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