Fix MD013: wrap long line in r.gwflow.md

Author: Rhythm Patel

raster/r.gwflow/r.gwflow.md

@@ -41,7 +41,8 @@ $$
 In detail for 2 dimensions:
 
 $$
-\frac{dh}{dt} \cdot S = K_{xx} \frac{\partial^2 h}{\partial x^2} + K_{yy} \frac{\partial^2 h}{\partial y^2} + q
+\frac{dh}{dt} \cdot S = K_{xx} \frac{\partial^2 h}{\partial x^2} + 
+K_{yy} \frac{\partial^2 h}{\partial y^2} + q
 $$
 
 - $h$ -- the piezometric head in [m]