Update Lec1.agda and Exe1.agda

[shubhamkumar13]

The current function endoFunctorVec

endoFunctorVec : ∀ {n} → EndoFunctor λ X → Vec X n
endoFunctorVec = applicativeEndoFunctor

generates the following error
Error :

No instance of type Applicative (λ X → Vec X n) was found in scope.
when checking that the expression applicativeEndoFunctor has type
EndoFunctor (λ X → Vec X n)

and it was due to the fact the it was looking for a instance of Applicative that satisfies the type Applicative (λ X → Vec X n) which is applicativeVec but since that is not specified by the instance keyword this particular function fails to generate the instance of EndoFunctor

This PR fixes that.