change li break to handle safari user stylesheet bug

Author: liz abinante!

modules/primer-markdown/lib/lists.scss

@@ -45,6 +45,10 @@
     margin-bottom: 0;
   }
 
+  li {
+    word-wrap: break-all;
+  }
+
   li > p {
     margin-top: $spacer-3;
   }