Add r prefix to the docstring of the SeldenMatobaThomsonSpectrum model

cherab/core/model/laser/model.pyx

@@ -30,15 +30,15 @@ from cherab.core.utility.constants cimport SPEED_OF_LIGHT, ELECTRON_CLASSICAL_RA
 
 
 cdef class SeldenMatobaThomsonSpectrum(LaserModel):
-    """
+    r"""
     Thomson Scattering based on Selden-Matoba.
 
     The class calculates Thomson scattering of the laser to the spectrum. The model of the scattered spectrum used is based on
     the semi-empirical model by Selden and the Thomson scattering cross-section is taken from Matoba articles. The spectral contribution
     of the scattered laser light c is calculated as a sum of contributions of all laser wavelengths
 
     .. math::
-         c(\\lambda) =  c r_e^2 n_e cos^2\\theta \\sum_{\\lambda_L} \\frac{E_L(\\lambda_l) S(\\frac{\\lambda}{\\lambda_L} - 1, \\varphi, T_e)}{\\lambda_L},
+         c(\lambda) =  c r_e^2 n_e cos^2\\theta \\sum_{\\lambda_L} \\frac{E_L(\\lambda_l) S(\\frac{\\lambda}{\\lambda_L} - 1, \\varphi, T_e)}{\\lambda_L},
     
 
     where :math:`\\lambda` is the spectrum's wavelength, :math:`r_e` is the classical electron radius, :math:`n_e` is the electron delsity,