The "Signature Algorithm" describes what kind of signature was used by Let's Encrypt's intermediate certificate to sign your leaf certificate. By default, unless you've specifically opted-in to get signed by an ECDSA intermediate, your certificates will be signed with the RSA intermediate. That's the "sha256WithRSAEncryption" you see. That's the same whether your key, used by your certificate to secure a connection, is an RSA or ECDSA key.
You'll want to look at the "key size" and "public key algorithm" and the like to see which kind of public key your certificate uses itself.