Fix doc example for json:encode/2

[williamthome]

This PR fixes/improves the confusing json:encode/2 example.

[github-actions]

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// Erlang/OTP Github Action Bot

[michalmuskala]

Why is it confusing? The shell allows defining functions - if you paste this example into erl, it will work:

$ erl
Erlang/OTP 27 [erts-15.2.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit] [dtrace]

Eshell V15.2.2 (press Ctrl+G to abort, type help(). for help)
1> encoder([{_, _} | _] = Value, Encode) -> json:encode_key_value_list(Value, Encode);
   encoder(Other, Encode) -> json:encode_value(Other, Encode).
ok
2> custom_encode(Value) -> json:encode(Value, fun(Value, Encode) -> encoder(Value, Encode) end).
ok
3> iolist_to_binary(custom_encode([{a, []}, {b, 1}])).
<<"{\"a\":[],\"b\":1}">>

[williamthome]

if you paste this example into erl, it will work:

Hmm, now I see. I was not aware of this capability. I'll close this PR. Thank you!

[dgud]

Hmm, now I see. I was not aware of this capability. I'll close this PR. Thank you!

It is rather new, came with the new shell, 27?

[garazdawi]

It is rather new, came with the new shell, 27?

26!