doc: Correct usage of `cassini-sdk.yml` in documentation

`cassini-sdk.yml` is one of two base configs for cassini (the other
being `cassini.yml`). As `cassini-sdk.yml` includes `cassini.yml` the
intended use is

`kas build \
meta-cassini-config/kas/cassini-sdk.yml:\
meta-cassini-config/kas/n1sdp.yml`

... rather than...

`kas build \
meta-cassini-config/kas/cassini.yml:\
meta-cassini-config/kas/cassini-sdk.yml:\
meta-cassini-config/kas/n1sdp.yml`

Update `Build, Deploy and Validate Cassini Image` to make this clearer

Signed-off-by: Adam Johnston <adam.johnston@arm.com>