Fix shell command in README

[ilkimo]

If applied, this commit will correct the yq command to extract the public key from the secret.

Changes done:

• fix the $ yq .stringData key path
• remove the "| base64 --decode" as the key is not encoded when generated by the $ flux create secret git command
• use the proper path to the file according to the previous command in the README step-by-step explanation

The proper use of the $ yq command can also be verified directly in the official help section $ flux create secret git --help
Launch it and search for "yq" to jump directly to the --help example.