Update index.d.ts with correct type for GetCellType

[glinford]

Fix type

Description

Update index.d.ts with correct type for GetCellType

Related Issue

N/A

Motivation and Context

N/A

How Has This Been Tested

In my project

Types of changes

• Docs change / refactoring / dependency upgrade
• 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 change)
• [x ] Type fix

Checklist

• [ x] My code follows the code style of this project.
• [ x] My change requires a change to the documentation.
• [ x] I have updated the documentation accordingly.
• [ x] I have read the CONTRIBUTING document.
• [ x] I have added tests to cover my changes.
• [ x] All new and existing tests passed.

[xuri]

LGTM, many thanks!