libssh2_knownhost_check docs: correct the prototype

docs/libssh2_knownhost_check.3

@@ -8,7 +8,8 @@ libssh2_knownhost_check - check a host+key against the list of known hosts
 #include <libssh2.h>
 
 int libssh2_knownhost_check(LIBSSH2_KNOWNHOSTS *hosts,
-                            char *host, char *key, size_t keylen,
+                            const char *host,
+                            const char *key, size_t keylen,
                             int typemask,
                             struct libssh2_knownhost **knownhost);
 .SH DESCRIPTION