docs: Correct image signing manual

As recent article https://blog.trailofbits.com/2024/02/16/a-few-notes-on-aws-nitro-enclaves-images-and-attestation
properly mentions we need to verify the signature of PCR0 rather then decrypt it
in the last step of our Image Signing manual

Signed-off-by: Eugene Koira <[email protected]>

docs/image_signing.md

@@ -58,7 +58,7 @@ contains an `EifSectionType`. You can find more details about these
 headers in the [eif-defs](https://github.com/aws/aws-nitro-enclaves-image-format/) crate.
 
 2. For each PCR Signature use the public key from the signing
-certificate to decrypt the payload from the `COSESign1` object
+certificate to verify the payload from the `COSESign1` object
 (this can be done using the following crate:
 [aws-nitro-enclaves-cose](https://github.com/awslabs/aws-nitro-enclaves-cose))
 and check that the PCR's value is the same as the one computed by