<Doc> correct the formula of Delta rho_R in scf_thr_type.

Author: sunliang98

docs/advanced/input_files/input-main.md

@@ -1173,7 +1173,7 @@ Note: In new angle mixing, you should set `mixing_beta_mag >> mixing_beta`. The
 - **Type**: Integer
 - **Description**: Choose the calculation method of convergence criterion.
   - **1**: the criterion is defined as $\Delta\rho_G = \frac{1}{2}\iint{\frac{\Delta\rho(r)\Delta\rho(r')}{|r-r'|}d^3r d^3r'}$.
-  - **2**: the criterion is defined as $\Delta\rho_R = \int{|\Delta\rho(r)|d^3r}$.
+  - **2**: the criterion is defined as $\Delta\rho_R = \frac{1}{N_e}\int{|\Delta\rho(r)|d^3r}$, where $N_e$ is the number of electron.
 
   Note: This parameter is still under testing and the default setting is usually sufficient.