Fix index

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

hm.tex

@@ -234,7 +234,7 @@ \chapter*{Preface}
   This is a scalar quantity that, as we will see, measures its resistance to changes in its state of motion, also known as \emphidx{inertia}.
 
   \begin{tcolorbox}[title=Newton's second law of motion]
-    \index{Newton ! second law}
+    \index{Newton!second law}
     There exist \emphidx{frames of reference}, that is, systems of coordinates, in which the motion of the particle is described by a differential equation involving the forces $\vb*{F}$ acting on the point particle, its mass $m$ and its acceleration as follows
     \begin{equation}\label{eq:newton}
       \vb* F = m \ddot{\vb*{x}}.
@@ -298,7 +298,7 @@ \chapter*{Preface}
     In this model of the solar system, with the Sun fixed at the origin, we will describe the Earth by a point particle of mass $m$ whose position (and motion) is described by a vector $\vb*{x}\in\mathbb{R}^3$.
 
     Due to our choice of coordinates, the gravitational attraction of the Sun acts in the direction of $-\vb*{x}(t)$.
-    \emph{Newton's law of universal gravitation} \index{Newton ! universal gravitation} says that such a force is proportional to
+    \emph{Newton's law of universal gravitation} \index{Newton!universal gravitation} says that such a force is proportional to
     \begin{equation}
       \frac{GmM}{\|\vb*{0}-\vb*{x}\|^2} = \frac{GmM}{\|\vb*{x}\|^2},
     \end{equation}