Fix mocking example in README.md

[stufro]

When running the full example from the Mocks and Doubles section, the following error occurs from the last line of the example:

Error: Undefined local variable or method 'thing'

I realised that this should actually be dbl, changing it to this makes the test pass.

[icy-arctic-fox]

:facepalm: Ugh, nice catch.