Reference to dropped create-credentials-encryption-key in INSTALL

[cfi-gb]

The https://github.com/greenbone/gvmd/blob/master/INSTALL.md file currently contains the following statement:

gvmd --create-credentials-encryption-key

It seems this was dropped some time ago with 10f0632cca096965a0261928fc1dc04a90ebe0dc and the text should be updated accordingly to point out that the manager needs to be restarted to re-generate the keys.

See: https://community.greenbone.net/t/cant-create-new-credential-encryption-key-via-openvasmd/1019/4