DOC: fix CSS bug

doc/themes/scikit-learn/static/nature.css_t

@@ -870,11 +870,14 @@ div.carousel>.carousel-inner>.item {
 
 div.carousel img {
     max-height: 70px;
-    max-width: 50%;
     background-color: #fff;
     margin: auto;
 }
 
+div#testimonials_carousel img {
+    max-width: 50%;
+}
+
 div#testimonials_carousel {
     height: 120px;
 }