Update spec.md to avoid suggesting ! is a binary operator

[ascopes]

The current spec appears to suggest that the bang is a binary operator, which would enable expressions such as foo ! bar.

https://github.com/hashicorp/hcl/blob/main/hclsyntax/parser.go#L1070 seems to suggest this is only a unary operator.

[hashicorp-cla]

All committers have signed the CLA.

[alisdair]

Thanks!