fix comment

Author: dderjoel

src/field_5x52_int128_impl.h

@@ -68,10 +68,10 @@ static FIAT_SECP256K1_DETTMAN_FIAT_INLINE secp256k1_uint128 u128_add_u128_u128(s
   /* adding low b to r*/
   secp256k1_u128_accum_u64(&r, bl);
 
-  /* adding high b*/
   rl = secp256k1_u128_to_u64(&r);
   rh = secp256k1_u128_hi_u64(&r);
 
+  /* adding high b*/
   bh = secp256k1_u128_hi_u64(&b);
   rh += bh;