[docs] Fix typo fierst to first

docs/Generics/chapters/building-generic-signatures.tex

@@ -628,7 +628,7 @@ \section{Requirement Validity}\label{generic signature validity}
 \ldots\vdash \ConfReq{Self.Iterator}{IteratorProtocol}\tag{1}\\
 (1)\vdash \texttt{Self.Iterator.Element}\tag{2}
 \end{gather*}
-Intuitively, anything we can say about the protocol \texttt{Self} type in this signature, is also true of an arbitrary type parameter \texttt{T.A.B} in some other generic signature $G$ where we can fierst derive the conformance requirement $\ConfReq{T.A.B}{Sequence}$. So a particular consequence of the above is that there ought to be a derivation of a valid type parameter \texttt{T.A.B.Iterator.Element} in $G$. We can show that this derivation always exists.
+Intuitively, anything we can say about the protocol \texttt{Self} type in this signature, is also true of an arbitrary type parameter \texttt{T.A.B} in some other generic signature $G$ where we can first derive the conformance requirement $\ConfReq{T.A.B}{Sequence}$. So a particular consequence of the above is that there ought to be a derivation of a valid type parameter \texttt{T.A.B.Iterator.Element} in $G$. We can show that this derivation always exists.
 
 We do this with a \IndexDefinition{formal substitution}\emph{formal substitution}. When writing down derivations, we've been doing formal substitution already. Each kind of derivation step is defined in the form of a ``schema,'' where it is understood the various meta-syntactic variables (\texttt{T} for type parameter, \texttt{P} for protocol, \texttt{A} for associated type, and so on) are replaced with concrete instances of those entities in some specific generic signature $G$. We can also imagine applying a formal substitution to an existing derivation, replacing some elements in a way that preserves the validity of the derivation.