Wrong parameter name in CONTRIBUTION GUIDE

The parameter name in the [CONTRIBUTION GUIDE](https://github.com/Azure/azure-quickstart-templates/tree/master/1-CONTRIBUTION-GUIDE#deploying-samples) for artifact staging directory is wrong, the correct one should be "-ArtifactStagingDirectory", without "s" after "Artifact".

```powershell
.\Deploy-AzureResourceGroup.ps1 -ResourceGroupLocation 'eastus' -ArtifactsStagingDirectory '[foldername]'
```
```powershell
.\Deploy-AzureResourceGroup.ps1 -ResourceGroupLocation 'eastus' -ArtifactsStagingDirectory '201-vm-custom-script-windows' -UploadArtifacts
```

Open the Deploying Samples section of 1-CONTRIBUTION-GUIDE and compare the documented parameter with Deploy-AzureResourceGroup.ps1 usage. Replace the incorrect parameter name in both shown PowerShell examples, then verify that the guide consistently uses the corrected name.

Written by the indexing model from the issue text.