Update Package-Manager-Console-PowerShell-Reference.markdown

Get-Package summary contradicted itself in describing how the -ListAvailable switch was working.

See: http://nuget.codeplex.com/SourceControl/latest#src/VsConsole/PowerShellCmdlets/GetPackageCommand.cs
Merge pull request #196 from felickz/patch-1

Update Package-Manager-Console-PowerShell-Reference.markdown