fix assignments example (#48)

Author: scottnicolson

Diff for: source/includes/20170710/resources/_assignments.md.erb

@@ -124,7 +124,7 @@ Name | Data Type | Description
 
 If the date time now is November 11, 2017 8:42:00 AM UTC:
 
-`<%= current_page.data.api_root_url %>/assignments?available_after=2017-11-11T10:42:00Z`
+`<%= current_page.data.api_root_url %>/assignments?available_before=2017-11-11T10:42:00Z`
 
 #### Level 8 and 42 Kanji Assignments Which Have Been Burned