vfmt: change all '$expr' to '${expr}' (#16428)

vlib/context/README.md

@@ -73,7 +73,7 @@ fn main() {
 	ch := gen(mut ctx2)
 	for i in 0 .. 5 {
 		v := <-ch
-		println('> received value: $v')
+		println('> received value: ${v}')
 		assert i == v
 	}
 	println('> main is finished here')
@@ -187,4 +187,4 @@ fn main() {
 	assert value == dump(f(ctx, key))
 	assert not_found_value == dump(f(ctx, 'color'))
 }
-```
+```