curl_formadd.3: fixed wrong "end-marker" syntax

Reported and fixed by: Eric S. Raymond
Bug: http://curl.haxx.se/bug/view.cgi?id=1233
libcurl-tutorial.3: remove incorrect backslash

A single backslash in the content is not legal nroff syntax.

Reported and fixed by: Eric S. Raymond
Bug: http://curl.haxx.se/bug/view.cgi?id=1234