PascalCase parameter names

[felixfbecker]

The current casing is inconsistent:

Get-GitHubIssueForRepository -
repositoryUrl        closedOnOrAfter      Debug                ErrorVariable        OutBuffer            
state                closedOnOrBefore     ErrorAction          WarningVariable      PipelineVariable     
createdOnOrAfter     gitHubAccessToken    WarningAction        InformationVariable  
createdOnOrBefore    Verbose              InformationAction    OutVariable 

[HowardWolosky]

Agreed. Will fix.