fix typo in docs (#545)

Author: sbryngelson

docs/documentation/case.md

@@ -439,7 +439,7 @@ Practically, `weno_eps` $<10^{-6}$ is used.
 - `riemann_solver` specifies the choice of the Riemann solver that is used in simulation by an integer from 1 through 3.
 `riemann_solver = 1`, `2`, and `3` correspond to HLL, HLLC, and Exact Riemann solver, respectively ([Toro, 2013](references.md#Toro13)).
 
-- `low_Mach` specifies the choice of the low Mach number correction scheme for the HLLC Riemann solver. `low_Mach = 0` is default value and does not apply any correction scheme. `low_Mach = 1` and `2` apply the anti-dissipation pressure correction method ([Chen et al., 2022](references.md#Chen22)) and the improved velocity reconstruction method ([Thornber et al., 2008](reference.md#Thornber08)). This feature requires `riemann_solver = 2` and `model_eqns = 2`.
+- `low_Mach` specifies the choice of the low Mach number correction scheme for the HLLC Riemann solver. `low_Mach = 0` is default value and does not apply any correction scheme. `low_Mach = 1` and `2` apply the anti-dissipation pressure correction method ([Chen et al., 2022](references.md#Chen22)) and the improved velocity reconstruction method ([Thornber et al., 2008](references.md#Thornber08)). This feature requires `riemann_solver = 2` and `model_eqns = 2`.
 
 - `avg_state` specifies the choice of the method to compute averaged variables at the cell-boundaries from the left and the right states in the Riemann solver by an integer of 1 or 2.
 `avg_state = 1` and `2` correspond to Roe- and arithmetic averages, respectively.