checker/parser: fix outdated error messages

vlib/v/checker/checker.v

@@ -335,7 +335,7 @@ fn (c mut Checker) assign_expr(assign_expr mut ast.AssignExpr) {
 			scope := c.file.scope.innermost(assign_expr.pos.pos)
 			if v := scope.find_var(it.name) {
 				if !v.is_mut {
-					c.error('`$it.name` is immutable, declare it with `mut`', assign_expr.pos)
+					c.error('`$it.name` is immutable, declare it with `var` to assign to it', assign_expr.pos)
 				}
 			}
 		}

vlib/v/parser/for.v

@@ -24,7 +24,7 @@ fn (var p Parser) for_stmt() ast.Stmt {
 			is_inf: true
 		}
 	} else if p.tok.kind in [.key_mut, .key_var] {
-		p.error('`mut` is not needed in for loops')
+		p.error('`var` is not needed in for loops')
 	} else if p.peek_tok.kind in [.decl_assign, .assign, .semicolon] || p.tok.kind == .semicolon {
 		// `for i := 0; i < 10; i++ {`
 		var init := ast.Stmt{}