[fix/docs-edit-username] Docs: change wording so it no longer suggests username is editable

[felix-schwarz]

Description

• docs: change wording so it no longer suggests username is editable

Related Issue

• reported via RocketChat

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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