Fix broken source link in expansion panels docs

[arxanas]

Description

Update link to the source code for expansion panels.

Motivation and Context

Resolves #8747

• Link is currently https://github.com/vuetifyjs/vuetify/tree/master/packages/vuetify/src/components/VExpansionPanels (broken)
• Should be https://github.com/vuetifyjs/vuetify/tree/master/packages/vuetify/src/components/VExpansionPanel

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 features but makes 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)
• I've added new examples to the kitchen (applies to new features and breaking changes in core library)