STYLE: use switch value for 'writeCompression' in controlDict's

- simpler/clearer to use "on/off" instead of "compressed/uncompressed"

tutorials/discreteMethods/dsmcFoam/freeSpacePeriodic/system/controlDict

@@ -37,13 +37,13 @@ writeFormat     ascii;
 
 writePrecision  10;
 
-writeCompression uncompressed;
+writeCompression off;
 
 timeFormat      general;
 
 timePrecision   6;
 
-runTimeModifiable yes;
+runTimeModifiable true;
 
 adjustTimeStep  no;