Fix broken image links on front page (dart-lang#5502)

Author: mit-mit

src/index.html

@@ -267,7 +267,7 @@
                                 </div>
                             </div>
                         </li>
-                        <li tabindex="0" data-banner="assets/dash/svg/1-2 language optimized.svg">
+                        <li tabindex="0" data-banner="assets/dash/svg/1-2-language-optimized.svg">
                             <div>
                                 <div class="bullet-container">
                                     <div class="animated-bullet"></div>
@@ -279,7 +279,7 @@
                                 </div>
                             </div>
                         </li>
-                        <li tabindex="0" data-banner="assets/dash/svg/1-3 familiar syntax.svg">
+                        <li tabindex="0" data-banner="assets/dash/svg/1-3-familiar-syntax.svg">
                             <div>
                                 <div class="bullet-container">
                                     <div class="animated-bullet"></div>