Updated README.md to reflect `sha256` deprecation in favor of `identi…

[CallVoidStar]

I configured moroz with a binary rule following the current examples in README.md. I pasted the sha256 from the block event into my config:

[[rules]]
rule_type = "BINARY"
policy = "ALLOWLIST"
sha256 = "<snip>"

I received an error on a Santa client after running santactl sync:

Ignoring bad rule: policy: ALLOWLIST
rule_type: BINARY

received 100 rules
processed 99 rules

After digging into why sha256 wasn't working and was actively rejected by Santa clients, I found this line of code in Santa project that deprecates sha256 in favor of the term identifier:

https://github.com/google/santa/blob/261425aa64bd49523ef99845dab04797da4fa614/Source/santasyncservice/syncv1.proto#L293

Using identifier instead fixed my issue. This change updates README.md to save others from having to track this down too.