Update mainchain registration example

[Tschakki]

What was the problem?

This PR partially resolves https://github.com/LiskHQ/lisk-docs/issues/1889

How was it tested?

https://github.com/LiskHQ/lisk-docs/pull/1890 https://liskhq.github.io/lisk-docs/beta/build-blockchain/register-sidechain.html

[codecov[bot]]

Codecov Report

Merging #9113 (c320adc) into release/6.1.0 (185e3fa) will decrease coverage by 0.01%. The diff coverage is n/a.

@@                Coverage Diff                @@
##           release/6.1.0    #9113      +/-   ##
=================================================
- Coverage          84.34%   84.34%   -0.01%     
=================================================
  Files                652      652              
  Lines              23966    23966              
  Branches            3489     3489              
=================================================
- Hits               20215    20214       -1     
- Misses              3751     3752       +1     

see 1 file with indirect coverage changes