Typo in session attribute

https://github.com/google/go-tpm/blob/master/tpm2/constants.go#L232
```golang
const (
AttrContinueSession SessionAttributes = 1 << iota
AttrAuditExclusive
AttrAuditReset
_ // bit 3 reserved
_ // bit 4 reserved
AttrDecrypt
AttrEcrypt /// <<< should be AttrEncrypt
AttrAudit
)
```

Open constants.go around line 232 and inspect the session attribute declarations. Confirm the misspelled AttrEcrypt identifier should be AttrEncrypt, then check the surrounding bit ordering and reserved entries. Done means the exported constant name matches the TPM session attribute spelling without changing the other declarations.

Written by the indexing model from the issue text.