docs: fix altsrc example

Author: ldez

docs/migrate-v2-to-v3.md

@@ -108,9 +108,11 @@ Shell command to find them all: `fgrep -rl github.com/urfave/cli/v2 *`
 
 === "v3"
 
+    Requires to use at least `github.com/urfave/cli-altsrc/[email protected]`
+
     ```go
     cli.StringFlag{
-        Sources: altsrc.JSON("key", "/tmp/foo")
+        Sources: cli.NewValueSourceChain(altsrcjson.JSON("key", altsrc.StringSourcer("/path/to/foo.json"))),
     }
     ```