docs: fix function typo in curl_easy_option_next.3

[rilysh]

Hello,
When I was reading libcurl docs, I noticed that there's a minor mistake in the curl_easy_option_next example manpage. In that specific example, curl_easy_option_by_next() has been used, however, it doesn't exist in libcurl.

URL: https://curl.se/libcurl/c/curl_easy_option_next.html

[bagder]

Thanks!