Fix cucumber steps description (s/should/must/)

features/session/termination.feature

@@ -3,4 +3,4 @@ Feature: program termination
   Scenario: terminates when requested to quit
     Given uhwm is running
     When I tell uhwm to quit
-    Then uhwm should terminate successfully
+    Then uhwm must terminate successfully

features/steps/run_steps.rb

@@ -27,6 +27,6 @@ Then /^the exit status must be (\d+)$/ do |exit_status|
   assert_exit_status exit_status.to_i
 end
 
-Then /^uhwm should terminate successfully$/ do
+Then /^uhwm must terminate successfully$/ do
   assert_exit_status 0
 end