Fixed "Example usage"

Example usage for adding API key on https://chocolatey.org/account is:
nuget.exe setApiKey my-API-key -Source http://chocolatey.org/

That could be:
nuget setApiKey my-API-key -Source http://chocolatey.org/

(nuget.exe does not exist, not in current version of standard instalation nuget.commandline, what suggests https://github.com/chocolatey/chocolatey/wiki/CommandsPush
Merge pull request #66 from marsjaninzmarsa/patch-1

Fixed "Example usage"