Fix ktlint violation in JsonMessage.kt

[bgrozev]

Summary

• #1308 added toString() overrides in JsonMessage.kt that trip ktlint's rule requiring a single-line expression body to stay on the function signature line, breaking CI on master
• Reformat the two toString() overrides to satisfy ktlint (ran ktlint-maven-plugin:format)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.86%. Comparing base (9c814ab) to head (ae51d1c).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...c/main/kotlin/org/jitsi/jicofo/xmpp/JsonMessage.kt	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1309      +/-   ##
==========================================
- Coverage   52.88%   52.86%   -0.02%     
==========================================
  Files         133      133              
  Lines        8511     8514       +3     
  Branches     1293     1296       +3     
==========================================
  Hits         4501     4501              
- Misses       3384     3387       +3     
  Partials      626      626              

Files with missing lines	Coverage Δ
...c/main/kotlin/org/jitsi/jicofo/xmpp/JsonMessage.kt	88.63% <0.00%> (-6.49%)	⬇️

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