[AppService]: fix Azure#20746 New-AzWebAppCertificate -hostname indicates support for multiple hostnames (Azure#20790)

Author: Kotasudhakarreddy

src/Websites/Websites/Cmdlets/Certificates/NewAzureWebAppCertificate.cs

@@ -59,7 +59,7 @@ public class NewAzureWebAppCertificate : WebAppBaseClientCmdLet
         [ValidateNotNullOrEmpty]
         public string Slot { get; set; }
 
-        [Parameter(ParameterSetName = ParameterSet1Name, Position = 3, Mandatory = true, HelpMessage = "Custom hostnames associated with web app/slot.")]
+        [Parameter(ParameterSetName = ParameterSet1Name, Position = 3, Mandatory = true, HelpMessage = "Custom hostname associated with web app/slot.")]
         [ValidateNotNullOrEmpty]
         public string HostName { get; set; }
 

src/Websites/Websites/help/New-AzWebAppCertificate.md

@@ -77,7 +77,7 @@ Accept wildcard characters: False
 ```
 
 ### -HostName
-Custom hostnames associated with web app/slot.
+Custom hostname associated with web app/slot.
 
 ```yaml
 Type: System.String