Fix typo (#69)

Author: superbrothers

test/helper.bash

@@ -11,7 +11,7 @@ assert_success() {
 
 assert_failure() {
   if [[ "$status" == 0 ]]; then
-    echo "expected: non-zero exit codde"
+    echo "expected: non-zero exit code"
     echo "actual: $status"
     echo "output: $output"
     return 1