parser: sugggest map initialization with the correct syntax

vlib/v/parser/parse_type.v

@@ -163,7 +163,9 @@ fn (mut p Parser) parse_map_type() ast.Type {
 	p.check(.rsbr)
 	if p.tok.kind == .lsbr {
 		if p.peek_tok.kind !in [.rsbr, .number] {
-			p.error_with_pos('maps can only have a single key', p.peek_tok.pos())
+			s := p.table.type_to_str(key_type)
+			p.error_with_pos('maps can only have a single key. To declare a map use `map[${s}]${p.peek_tok.lit}{}` instead',
+				p.peek_tok.pos())
 			return 0
 		}
 	}

vlib/v/parser/tests/map_key_twice_err.out

@@ -1,4 +1,4 @@
-vlib/v/parser/tests/map_key_twice_err.vv:1:14: error: maps can only have a single key
+vlib/v/parser/tests/map_key_twice_err.vv:1:14: error: maps can only have a single key. To declare a map use `map[u8]u8{}` instead
     1 | _ := map[u8][u8]{}
       |              ~~
     2 |