minor #18576 [Messenger] Fix the introduction diagram (javiereguiluz)

This PR was merged into the 6.2 branch.

Discussion
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[Messenger] Fix the introduction diagram

Fixes #17973.

I know that the font doesn't look the same ... but I cannot make it work. I'll keep trying to see if I fix this, but I recommend to just merged this "as is" now to fix the bug meanwhile. Thanks.

Commits
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8bbaf21 [Messenger] Fix the introduction diagram

Author: javiereguiluz