Small change: remove unintentional whitespace (#34418)

On other challenges, only the affected places would have a placeholder whitespace, if there is one. Removed the unnecessary ones from this challenge.

curriculum/challenges/english/01-responsive-web-design/css-flexbox/use-display-flex-to-position-two-boxes.english.md

@@ -44,14 +44,12 @@ tests:
     background-color: dodgerblue;
     width: 50%;
     height: 50%;
-
   }
 
   #box-2 {
     background-color: orangered;
     width: 50%;
     height: 50%;
-
   }
 </style>
 <div id="box-container">