the invalid variable syntax error message was misleading. changed text a little bit (#11953)

terraform/interpolate.go

@@ -306,10 +306,10 @@ func (i *Interpolater) valueSimpleVar(
 	// relied on this for their template_file data sources. We should
 	// remove this at some point but there isn't any rush.
 	return fmt.Errorf(
-		"invalid variable syntax: %q. If this is part of inline `template` parameter\n"+
+		"invalid variable syntax: %q. Did you mean 'var.%s'? If this is part of inline `template` parameter\n"+
 			"then you must escape the interpolation with two dollar signs. For\n"+
 			"example: ${a} becomes $${a}.",
-		n)
+		n, n)
 }
 
 func (i *Interpolater) valueTerraformVar(