Fix typo

Signed-off-by: Marcello Seri <[email protected]>

2-tangentbdl.tex

@@ -828,7 +828,7 @@ \section{The tangent bundle}\label{sec:tangentbundle}
     \widetilde\psi\circ & \widetilde\varphi^{-1}\left(x^1, \ldots, x^n, v^1, \ldots, v^n\right)                                                            \\
                         & =\left(y^1(p),\ldots, y^n(p), \frac{\partial y^1}{\partial x^j}(p) v^j, \ldots, \frac{\partial y^n}{\partial x^j}(p) v^j\right),
   \end{align}
-  where $p = \phi^{-1}(x)$, which is clearly smooth.
+  where $p = \varphi^{-1}(x)$, which is clearly smooth.
 
   \begin{figure*}[htp]
     \includegraphics{2_7-tg_bdl_coord.pdf}