Fix placeholder color for search in dark mode

Author: pwizla

docusaurus/src/scss/custom-search-bar.scss

@@ -335,7 +335,7 @@
         color: var(--strapi-dark-100);
 
         .docsearch-btn-placeholder {
-          color: var(--strapi-dark-100);
+          color: var(--strapi-neutral-400);
         }
 
         &:hover {