Update README.md so that the key name become consistent (#215)

Signed-off-by: Kay Gosho <ketsume0211@gmail.com>

README.md

@@ -34,7 +34,7 @@ npx --yes @fastify/secure-session > secret-key
 If you don't want to use `npx`, you can still generate the `secret-key` installing the `@fastify/secure-session` library with your choice package manager, and then:
 
 ```sh
-./node_modules/@fastify/secure-session/genkey.js > secret_key
+./node_modules/@fastify/secure-session/genkey.js > secret-key
 ```
 
 Then, register the plugin as follows: