Readme - new args for the using() example

[mastazi]

Going trough the Readme, it made me think that senderId and recipientId were something specific to the redlock paradigm that I needed to know, instead they are just generic identifiers for the names of the locks, since this is a generic example I think it's better to use something generic like "foo" or "bar".

This is also in line with the next part of the readme, the "acquire" example which uses the string "a" which is clearly a generic identifier