typo

Signed-off-by: Riccardo Finotello <riccardo.finotello@gmail.com>

sec/part1/introduction.tex

@@ -42,7 +42,7 @@ While Nambu and Goto's formulation is fairly direct in its definition, it is usu
   \eta_{\mu\nu}.
   \label{eq:conf:polyakov}
 \end{equation}
-The \eom for the string $X^{\mu}\qty(\tau, \sigma)$ is therefore:
+The \eom for the string $X^{\mu}\qty(\tau, \sigma)$ are therefore:
 \begin{equation}
   \frac{1}{\sqrt{- \det \gamma}}\,
   \ipd{\alpha}