Update mix to match reference implementation ver

Reference implementation: https://extgit.iaik.tugraz.at/krypto/hadeshash

Tested with `sage code/poseidonperm_x5_254_3.sage` for inputs:
`[1,2,0]`, equivalent to using `circomlib/poseidon.js` with inputs
`[1,2]`, both return as result
`11309872961022349216464221841186646423561022368884850929991258903497301047946`

Tested with `sage code/poseidonperm_x5_254_5.sage` for inputs:
`[1,2,3,4,0]`, equivalent to using `circomlib/poseidon.js` with inputs
`[1,2,3,4]`, both return as result
`18181515143627462196415302348515936346022476168236332098176009127325427112991`

circuits/poseidon.circom

@@ -30,7 +30,7 @@ template Mix(t, M) {
     for (var i=0; i<t; i++) {
         lc = 0;
         for (var j=0; j<t; j++) {
-            lc += M[j][i]*in[j];
+            lc += M[i][j]*in[j];
         }
         out[i] <== lc;
     }