Fixing typo in line 14

[samdickerman]

The v-slot should be "dayHeader" not "dayHeadere".

This typo prevents the all day events from displaying at the top when using the example code.

I'm a total GitHub newbie so I apologize if I didn't do this right!

Description

Motivation and Context

How Has This Been Tested?

Markup:

// Paste your FULL Playground.vue here

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 not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)