port documentation spelling fixes to comments or strings in source code.

src/USER-NETCDF/dump_netcdf.cpp

@@ -785,7 +785,7 @@ void DumpNetCDF::write_header(bigint n)
       cell_angles[2] = acos(cosgamma)*180.0/MY_PI;
     }
 
-    // Recent AMBER conventions say that nonperiodic boundaries should have
+    // Recent AMBER conventions say that non-periodic boundaries should have
     // 'cell_lengths' set to zero.
     for (int dim = 0; dim < 3; dim++) {
         if (!domain->periodicity[dim])