Remove expression attributes from stability policy (#654)

Author: eemeli

spec/README.md

@@ -84,7 +84,7 @@ A reference to a _term_ looks like this.
 Updates to this specification will not change
 the syntactical meaning, the runtime output, or other behaviour
 of valid messages written for earlier versions of this specification
-that only use functions and expression attributes defined in this specification.
+that only use functions defined in this specification.
 Updates to this specification will not remove any syntax provided in this version.
 Future versions MAY add additional structure or meaning to existing syntax.