Update warning regarding expiration settings

[ibrahima]

Based on https://github.com/nateware/redis-objects/pull/213 and the comments on https://github.com/nateware/redis-objects/commit/4854553f451c1b983ce620e205c45acea1fefc5b, the current README incorrectly describes an issue with the expiration setting when it is actually meant to be describing an issue with the expireat setting. This updates the documentation to correctly describe the issue.

[matthewhively]

The way the documentation is worded, it almost seems like it should have a "bad example" and a "good example". So the wording is warning you about something, that isn't shown... since the example is already using lambda.