Fixed the way a string of an user is resolved

[lipkau]

Description

When Remove-JiraGroupMember uses a [String] for -User, Get-JiraUser does not resolve properly resolve the input.
This is fixed by using -UserName instead of -InputObject on Get-JiraUser

Motivation and Context

closes #299

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)

Checklist:

• My code follows the code style of this project.
• I have added Pester Tests that describe what my changes should do.
• I have updated the documentation accordingly.

[FriedrichWeinmann]

Looking good!