scope a.button to .promo, to remove an !important

(a.button is not used anywhere else; introduced in #1757)

Author: Fil

docs/.vitepress/theme/ObservablePromo.vue

@@ -62,15 +62,15 @@ h1 {
   opacity: 0.7;
 }
 
-a.button {
+.promo a.button {
   display: inline-block;
   border: 1px solid transparent;
   text-align: center;
   text-decoration: none;
   font-weight: 600;
   white-space: nowrap;
   background-color: var(--vp-c-neutral);
-  color: var(--vp-c-neutral-inverse) !important;
+  color: var(--vp-c-neutral-inverse);
   border-radius: 20px;
   padding: 0 20px;
   line-height: 38px;