Update README to fix bad paths and input/output

[dcousens]

This pull request updates the README to fix some bad paths (the example didn't work) and aligns the input with the output.

Additionally, rather than hinting that AGE_TPM_PIN is how you might use a pin, I update the copy to say that this is how you might use the plugin non-interactively, instead.

--pin will prompt when encrypting and decrypting, no environment variable required.

[dcousens]

If desired, I can split this up into two different pull requests, fixes and new copy

[Foxboron]

Looks good to me, thanks :)