s/\.NF/.nf/ to fix wrong macro name caught by man --warnings

docs/libssh2_userauth_publickey.3

@@ -6,7 +6,7 @@ libssh2_userauth_publickey - authenticate using a callback function
 .SH SYNOPSIS
 #include <libssh2.h>
 
-.NF
+.nf
 int libssh2_userauth_publickey(LIBSSH2_SESSION *session,
                                const char *user,
                                const unsigned char *pubkeydata,