Update user.adoc - Example of adding a 'secret text' credential and correction of a misspelling.

[kovacstim]

[MarkEWaite]

I don't think that example will have the desired result. When Jenkins creates the credentials.xml file, it encrypts the secret rather than storing it in plain text. Creating a valid credentials.xml file with content that is usable from inside Jenkins requires more than the plain text of the secret embedded in the credentials.xml file.