docs(readme): fix quickstart demo API key to match server hashing (#100)

The plaintext (`recotem-quickstart`, 18 chars) was below
`_API_KEY_MIN_LEN = 32`, and the published hash was a plain
sha256 instead of the scrypt KDF the server actually uses, so the
quickstart `curl /predict/...` always returned `invalid_api_key`.

Replace with a 32-char plaintext and the matching
`scrypt(N=2, r=8, p=1, salt=b"recotem.api-key.v1")` hex digest.

Author: marevol

README.md

@@ -76,8 +76,8 @@ artifact directory all in one place. Train a TopPop recommender from a
 # 1. Set demo keys. DEMO ONLY — for production, generate fresh keys with
 #    `recotem keygen --type signing` and `recotem keygen --type api`.
 export RECOTEM_SIGNING_KEYS="dev:0000000000000000000000000000000000000000000000000000000000000000"
-export RECOTEM_API_PLAINTEXT="recotem-quickstart"
-export RECOTEM_API_KEYS="dev:sha256:921281c95ab79adecf21d410f8c93d38d74b0c3c267c221c8291771de8e7c359"
+export RECOTEM_API_PLAINTEXT="recotem-quickstart-demo-key-0000"
+export RECOTEM_API_KEYS="dev:sha256:21be5c3be85b8d68123df9f9b6a26d8e307db30350ea8bcc844883e22ebcf125"
 
 # 2. Train, serve
 recotem train examples/quickstart/recipe.yaml