Polynomial: background of formula in advanced section

In the file `Polynomial/group__PkgPolynomialFunctions.html#ga0148f1612807b08b433f96c3ce07f2be` we see for the function `number_of_real_roots` in the advanced section that all formulas except 1 are nicely shown as (MathJax) formulas and that one is an image of which the background color is incorrect.
Replacing the image with a proper formula makes all consistent and proper.

Polynomial/doc/Polynomial/CGAL/polynomial_utils.h

@@ -494,8 +494,7 @@ For a sequence \f$ I:=(a_0,\ldots,a_n)\f$ of real numbers with \f$ a_0\neq 0\f$,
 \f[ C(I)=\ccSum{i=1}{s}\epsilon_i \f]
 where \f$ s\f$ is the number of subsequences of \f$ I\f$ of the form
 
-\image html underbrace.png
-\image latex underbrace.png
+\f[ (a,\underbrace{0, \dots,0}_{k},b) \f]
 
 with \f$ a\neq 0,b\neq 0, k\geq 0\f$.
 
@@ -546,8 +545,7 @@ For a sequence \f$ I:=(a_0,\ldots,a_n)\f$ of real numbers with \f$ a_0\neq 0\f$,
 \f[ C(I)=\ccSum{i=1}{s}\epsilon_i \f]
 where \f$ s\f$ is the number of subsequences of \f$ I\f$ of the form
 
-\image html underbrace.png
-\image latex underbrace.png
+\f[ (a,\underbrace{0, \dots,0}_{k},b) \f]
 
 with \f$ a\neq 0,b\neq 0, k\geq 0\f$.