Fixed typo in link to writing NHS messages

[stacey-budd]

Description

Fixed type in link on https://notify.nhs.uk/using-nhs-notify/formatting/bullet-points-numbered-lists.

Context

Link inaccurately reads 'Find out more about writing messages...'

Find out more about writing messages (NHS Digital service manual).

Corrected to read 'Find out more about writing NHS messages...'

Find out more about writing NHS messages (NHS Digital service manual).

Type of changes

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

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.