man page: add missing function argument

for libssh2_userauth_publickey_fromfile_ex()

Reported-by: "pastey"

Fixes #262

docs/libssh2_userauth_publickey_fromfile_ex.3

@@ -7,6 +7,7 @@ libssh2_userauth_publickey_fromfile - authenticate a session with a public key,
 .nf
 int libssh2_userauth_publickey_fromfile_ex(LIBSSH2_SESSION *session,
                                            const char *username,
+                                           unsigned int ousername_len,
                                            const char *publickey,
                                            const char *privatekey,
                                            const char *passphrase);