(graphviz) fix typo preventing uninstallation

[bluebird75]

Fixes #2512

Description

Typo in the powershell script, the code made no sense and would crash.

Motivation and Context

Uninstallation was not possible due to crash of unintall powershel script

How Has this Been Tested?

The fixed script was run successfully and uninstallation succeeded

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 change)
• Migrated package (a package has been migrated from another repository)

Checklist:

• My code follows the code style of this repository.
• My change requires a change to documentation (this usually means the notes in the description of a package).
• I have updated the documentation accordingly (this usually means the notes in the description of a package).
• I have updated the package description and it is less than 4000 characters.
• All files are up to date with the latest Contributing Guidelines
• The added/modified package passed install/uninstall in the Chocolatey Test Environment(https://github.com/chocolatey-community/chocolatey-test-environment/). Note that we don't support the use of any other environment.
• [ x] The changes only affect a single package (not including meta package).

[AppVeyorBot]

✅ Package verification completed without issues. PR is now pending human review

[bluebird75]

The error was also preventing the removal of shims

[AppVeyorBot]

✅ Package verification completed without issues. PR is now pending human review

[AdmiringWorm]

LGTM

[AdmiringWorm]

@bluebird75 your changes have been merged. Thanks for your contribution 👍