Fix desynonymize message

[CarolineDenis]

Fixes #4477

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone

Testing instructions

• Open a tree
• Synonymies 2 nodes i.e: A and B
• Click on the synonym node
• Click desynonymize node action
• Verify that the message in the dialog is " "A" will no longer be a synonym of "B"." instead of "A" will no longer be a synonym of "A".

[grantfitzsimmons]

• Verify that the message in the dialog is " "A" will no longer be a synonym of "B"." instead of "A" will no longer be a synonym of "A".

[emenslin]

• Verify that the message in the dialog is " "A" will no longer be a synonym of "B"." instead of "A" will no longer be a synonym of "A".