update README with NEGF equations

README.md

@@ -115,7 +115,7 @@ where $v=\mathrm{d} E /(\hbar \mathrm{d} k) = -2 a t / \hbar \sin (k a)$ so that
 The in-scattering terms
 
 $$
-\Sigma^\mathrm{in}_{1,2} = \Gamma \cdot f
+\Sigma^\mathrm{in}_{1,2} = \Gamma_1 \cdot f_1
 $$
 
  where $f_i$ is the Fermi-Dirac distribution function for contact $i$.