fix some spelling (#199)

Author: zingale

pyro/incompressible/simulation.py

@@ -87,7 +87,7 @@ def preevolve(self):
         self.cc_data.fill_BC("y-velocity")
 
         # 1. do the initial projection.  This makes sure that our original
-        # velocity field satisties div U = 0
+        # velocity field satisfies div U = 0
 
         # next create the multigrid object.  We want Neumann BCs on phi
         # at solid walls and periodic on phi for periodic BCs

pyro/lm_atm/simulation.py

@@ -197,7 +197,7 @@ def preevolve(self):
         self.cc_data.fill_BC("y-velocity")
 
         # 1. do the initial projection.  This makes sure that our original
-        # velocity field satisties div U = 0
+        # velocity field satisfies div U = 0
 
         # the coefficient for the elliptic equation is beta_0^2/rho
         coeff = 1/rho

pyro/multigrid/examples/project_periodic.py

@@ -20,7 +20,7 @@
 projection uses the divergence of the velocity field from the previous
 iteration as its source term.
 
-Note: the output file created stores the original field, the poluted
+Note: the output file created stores the original field, the polluted
 field, and the recovered field.
 """