fix relative path problem in cold traps

external/lab-equipment/cold-traps/cold-traps.md

 ---
 layout: page
-permalink: /external/lab-equipment/cold-traps
+permalink: /external/lab-equipment/cold-traps/
 shortcut: lab:cold-traps
 redirect_from:
   - /cards/lab-equipment:maintenance_of_cold_traps
@@ -89,4 +89,4 @@ When necessary, the acid cartridge (LIMS ID: 10608) should be exchanged
 * Remove the cartridge and replace it with the new one  
 * Close back the container  
 * Place the old cartridge in the chemical hood for evaporation in a retention container  
-* 24 hours later, trash it in a blue bin UN3291  
\ No newline at end of file
+* 24 hours later, trash it in a blue bin UN3291