fix typo (Plonky3#383)

fix typo in pcs open

fri/src/two_adic_pcs.rs

@@ -212,7 +212,7 @@ where
         - Instead of computing all alpha^i, we just compute alpha^i for i up to the largest width
         of a matrix, then multiply by an "alpha offset" when accumulating.
               a^0 x0 + a^1 x1 + a^2 x2 + a^3 x3 + ...
-            = a^0 ( a^0 x0 + a^1 x1 ) + a^2 ( a^0 x0 + a^1 x1 ) + ...
+            = a^0 ( a^0 x0 + a^1 x1 ) + a^2 ( a^0 x2 + a^1 x3 ) + ...
             (see `alpha_pows`, `alpha_pow_offset`, `num_reduced`)
 
         - For each unique point z, we precompute 1/(X-z) for the largest subgroup opened at this point.