hclsyntax: Correct spec to allow colons as object element delimiter

To allow easir adaptation of data already serialized as JSON, HCL native
syntax allows both equals signs _and_ colons for object constructors.

This was already implemented, but not reflected in the pseudo-BNF in
the specification.

hclsyntax/spec.md

@@ -273,7 +273,7 @@ tuple = "[" (
 object = "{" (
     (objectelem ("," objectelem)* ","?)?
 ) "}";
-objectelem = (Identifier | Expression) "=" Expression;
+objectelem = (Identifier | Expression) ("=" | ":") Expression;
 ```
 
 Only tuple and object values can be directly constructed via native syntax.