TUT: update snappy dictionaries to use minMedialAxisAngle

- canonical parameter name changed from "minMedianAxisAngle" to
  "minMedialAxisAngle" (DEC-2013), but was never previously reported
  as having changed.

tutorials/mesh/snappyHexMesh/gap_detection/system/snappyHexMeshDict

@@ -257,7 +257,7 @@ addLayersControls
 
     // Angle used to pick up medial axis points
     // Note: changed(corrected) w.r.t 16x! 90 degrees corresponds to 130 in 16x.
-    minMedianAxisAngle 90;
+    minMedialAxisAngle 90;
 
     // Create buffer region for new layer terminations
     nBufferCellsNoExtrude 0;