docs(/p/demo/json): update json package README.md (gnolang#2921)

<!-- please provide a detailed description of the changes made in this
pull request. -->
in the three examples provided in the README.md file, there's a useless
import of "fmt" package.

<details><summary>Contributors' checklist...</summary>

- [ ] Added new tests, or not needed, or not feasible
- [ ] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [ ] Updated the official documentation or not needed
- [ ] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [ ] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

examples/gno.land/p/demo/json/README.md

@@ -75,7 +75,6 @@ The converted `Node` type allows you to modify the JSON data or search and extra
 package main
 
 import (
-    "fmt"
     "gno.land/p/demo/json"
     "gno.land/p/demo/ufmt"
 )
@@ -100,7 +99,6 @@ Encoding (or Marshaling) is the functionality that converts JSON data represente
 package main
 
 import (
-    "fmt"
     "gno.land/p/demo/json"
     "gno.land/p/demo/ufmt"
 )
@@ -133,7 +131,6 @@ Here is an example of finding data with a specific key. For more examples, pleas
 package main
 
 import (
-    "fmt"
     "gno.land/p/demo/json"
     "gno.land/p/demo/ufmt"
 )