fix an issue about the unit

Author: sunml99

docs/examples/implicit-sol.md

@@ -23,7 +23,7 @@ nc_k                    0.00037
 - tau 
 
     The effective surface tension parameter, which describes the cavitation, the dispersion, and the repulsion interaction between the solute and the solvent that are not captured by the electrostatic terms.
-    We use the values of `tau`, `sigma_k`, `nc_k` that were obtained by a fit of the model to experimental solvation energies for molecules in water. tau = 0.525 meV/$Å^{2}$ = 1.0798e-05 Ry/$Bohr^{2}$.
+    We use the values of `tau`, `sigma_k`, `nc_k` that were obtained by a fit of the model to experimental solvation energies for molecules in water. tau = 0.525 $meV/Å^{2}$ = 1.0798e-05 $Ry/Bohr^{2}$.
 - sigma_k 
 
     We assume a diffuse cavity that is implicitly determined by the electronic structure of the solute. 

docs/input-main.md

@@ -1686,7 +1686,7 @@ This part of variables are used to control the usage of implicit solvation model
 #### tau
 
 - **Type**: Real
-- **Description**: The effective surface tension parameter, which describes the cavitation, the dispersion, and the repulsion interaction between the solute and the solvent that are not captured by the electrostatic terms. The unit is Ry/$Bohr^{2}$.
+- **Description**: The effective surface tension parameter, which describes the cavitation, the dispersion, and the repulsion interaction between the solute and the solvent that are not captured by the electrostatic terms. The unit is $Ry/Bohr^{2}$.
 - **Default**: 1.0798e-05
 
 #### sigma_k