Fix :first-child() / :last-child() CSS typos

29957e5 introduced a CSS typo in
twins.scss, where the first and last child pseudo-selectors were
followed by empty brackets. This worked at the time I wrote it
(perhaps the version of Sass we were using at that point handled the
mistake gracefully?) but our upcoming scorecards-2023 branches use
a new Sass compiler, which treats this as invalid CSS and causes
django pipeline to raise an exception.

Same goes for the :first-child() in the 2021 scorecards methodology.

caps/static/caps/scss/twins.scss

@@ -37,11 +37,11 @@
         background: rgba(255, 255, 255, 0.3);
     }
 
-    thead > tr:first-child() > :nth-child(2) {
+    thead > tr:first-child > :nth-child(2) {
         border-top: $table-border-width solid $table-border-color;
     }
 
-    tbody > tr:last-child() > :nth-child(2) {
+    tbody > tr:last-child > :nth-child(2) {
         border-bottom: $table-border-width solid $table-border-color;
     }
 

scoring/static/scoring/scss/methodology.scss

@@ -47,7 +47,7 @@
         }
 
         tbody tr td, thead tr th {
-            &:first-child() {
+            &:first-child {
                 text-align: left;
             }
         }