Correct a minor grammatical mistake ("a semisimple") (#249)

*Semisimple* is not a noun, I believe.

tex/rep-theory/semisimple.tex

@@ -297,7 +297,7 @@ \section{Density theorem}
 
 \section{Semisimple algebras}
 \begin{definition}
-	A finite-dimensional algebra $A$ is a \vocab{semisimple}
+	A finite-dimensional algebra $A$ is \vocab{semisimple}
 	if every finite-dimensional representation of $A$ is completely reducible.
 \end{definition}