typo in crypto_curve25519.c comment, spotted by rransom

src/common/crypto_curve25519.c

@@ -69,7 +69,7 @@ curve25519_secret_key_generate(curve25519_secret_key_t *key_out,
     return -1;
   if (extra_strong && !crypto_strongest_rand(k_tmp, CURVE25519_SECKEY_LEN)) {
     /* If they asked for extra-strong entropy and we have some, use it as an
-     * HMAC key to improve not-so-good entopy rather than using it directly,
+     * HMAC key to improve not-so-good entropy rather than using it directly,
      * just in case the extra-strong entropy is less amazing than we hoped. */
     crypto_hmac_sha256((char *)key_out->secret_key,
                     (const char *)k_tmp, sizeof(k_tmp),