inets: change uri_string:uri_string() type to string() where needed

[Alberdi]

Some parameters, such as ContentType in httpc:request/4, are wrongly documented as uri_string:uri_string(), but they don't allow binary values. This was introduced when migrating to the new documentation in commit 6108008

For example:

1> httpc:request(post, {<<"http://www.example.com">>, [], <<"application/xhtml+xml">>, <<>>}, [], []).
{error,invalid_request}

2> httpc:request(post, {<<"http://www.example.com">>, [], "application/xhtml+xml", <<>>}, [], []).
{ok,{{"HTTP/1.1",403,"Forbidden"}, [...]

[github-actions]

CT Test Results

  2 files   22 suites   9m 8s ⏱️
360 tests 353 ✅  7 💤 0 ❌
647 runs  587 ✅ 60 💤 0 ❌

Results for commit 63ad1ad.

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