Fix ind(D,d,x,y,p) in (i), (ii) and (iii) of Lemma 2.1.4

ind(D,d) : \Pi_{x,y : A} \Pi_{p : x = y} D(x,y,p) according to
Section 1.12.1 (Path induction).
first-order fix to the induction principle of V
unmarked errata for 6d9722d
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Merge pull request #693 from ahorn/lemma-2.1.4

Fix ind(D,d,x,y,p) in (i), (ii) and (iii) of Lemma 2.1.4
Merge pull request #698 from mikeshulman/Vind

first-order fix to the induction principle of V
Mark Errata (auto)