ICU-23169 Fixes two typos in datetime patterns

[yumaoka]

Fixed two typos in user guide

I duplicated changes in PR#3574 (#3574). The original PR commit had a difficulty to change commit message and squash commits.

Checklist

• Required: Issue filed: ICU-23169
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf