Typo in formula

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

sec/part1/dbranes.tex

@@ -125,7 +125,7 @@ that is we just need to consider the left coset where $R_{(t)}$ is a representat
   =
   \qty{ R_{(t)} \sim \cO_{(t)} R_{(t)} },
 \end{equation}
-where $\cO_{(t)} = \rS\qty( \OO{2} \times \OO{2} )$ is defined as
+where $\cO_{(t)} \in \rS\qty( \OO{2} \times \OO{2} )$ is defined as
 \begin{equation}
   \cO_{(t)}
   =