fix(tutorial): change command in sse-c tutorial

Author: luxifer

tutorials/object-storage-sse-c-with-secret-manager/index.mdx

@@ -33,9 +33,9 @@ With the following commands, you will create a key in the Key Manager, generate
 
 ```bash
 KEY_ID=$(scw keymanager key create -o template="{{.ID}}")
-DATA_KEY=$(scw keymanager key generate-data-key "$KEY_ID" -o json | jq -r .plaintext | base64 -d)
+scw keymanager key generate-data-key "$KEY_ID" -o json | jq -r .plaintext | base64 -d > ssec.key
 SECRET_ID=$(scw secret secret create name=ssec-key path=/keys -o template="{{.ID}}")
-scw secret version create "$SECRET_ID" data="$DATA_KEY"
+scw secret version create "$SECRET_ID" data="@ssec.key"
 ```
 
 ### Encryption key and digest preparation