Fix typo in Section 12.5 of the Open Logic Text

Author: FnControlOption

content/first-order-logic/axiomatic-deduction/deduction-theorem.tex

@@ -68,7 +68,7 @@
 is an axiom, then $\Gamma \Proves !B$. We also have that $\Gamma
 \Proves !B \lif (!A \lif !B)$ by \olref[prp]{ax:lif1}, and
 \olref{prop:mp} gives $\Gamma \Proves !A \lif !B$. If $!B \in \{ !A\}$
-then $\Gamma \Proves !A \lif !B$ because then last !!{sentence}~$!A
+then $\Gamma \Proves !A \lif !B$ because the last !!{sentence}~$!A
 \lif !B$ is the same as $!A \lif !A$, and we have !!{derive}d that in
 \olref[pro]{ex:identity}.