dhcpv6-server: T6381: fix typos in select ConfigError messages in current

[Giggum]

Change Summary

Fixes typos in select ConfigError messages (e.g. then -> than, multipe -> multiple)
This fixes are only needed in Current as they have already been applied to Sagitta through a seperate PR.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

https://vyos.dev/T6381

Related PR(s)

• dhcpv6-server: T3493: adds prefix range validation and fixes typos in… #3499

Component(s) name

Proposed changes

How to test

Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[c-po]

No backport required, already handled in:

• dhcpv6-server: T3493: adds prefix range validation and fixes typos in… #3499