Fix definition of Subseq(s,s').

As written it checks for identity between s_i and s_j, two indexes
of elements of the same sequence s, rather than indexes of elements
of two distinct sequences s and s'. This commit fixes that.

Author: beastaugh

content/incompleteness/arithmetization-syntax/proofs-in-nd.tex

@@ -288,7 +288,7 @@
   of~$\delta$. Any sequence of G\"odel numbers of sub-!!{derivation}s
   of~$\delta$ is a subsequence of it. Being a subsequence of is a
   primitive recursive relation: $\fn{Subseq}(s, s')$ holds iff
-  $\bforall{i<\len{s}}{\lexists[j<\len{s'}][(s)_i = (s)_j]}$. Being an
+  $\bforall{i<\len{s}}{\lexists[j<\len{s'}][(s)_i = (s')_j]}$. Being an
   immediate sub-!!{derivation} is as well: $\fn{Subderiv}(d, d')$ iff
   $\bexists{j<(d')_0}{d = (d')_j}$. So we can define
   $\fn{OpenAssum}(z, d)$ by