Fix doc

Switching function manual was disappeared

src/tools/SwitchingFunction.cpp

@@ -138,11 +138,11 @@ Notice that for backward compatibility we allow using `MATHEVAL` instead of `CUS
 Also notice that if the a `CUSTOM` switching function only depents on even powers of `x` it can be
 made faster by using `x2` as a variable. For instance
 \verbatim
-{CUSTOM FUNC=1/(1+x2^3) R_0=\f$r_0\f$ D_0=\f$d_0\f$}
+{CUSTOM FUNC=1/(1+x2^3) R_0=0.3}
 \endverbatim
 is equivalent to
 \verbatim
-{CUSTOM FUNC=1/(1+x^6) R_0=\f$r_0\f$ D_0=\f$d_0\f$}
+{CUSTOM FUNC=1/(1+x^6) R_0=0.3}
 \endverbatim
 but runs faster. The reason is that there is an expensive square root calculation that can be optimized out.