content/static: add whitespace to fix lint error

Change-Id: Ibd3e9dc0e07752b196905a8b5788e802b8bcb03b
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/286392
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>

content/static/css/unit_details.css

@@ -47,6 +47,7 @@
     scroll-margin-top: calc(var(--header-height) + 0.75rem);
   }
 }
+
 /*
 TODO(https://golang.org/issue/43773): fix highlight bug.
 .UnitDetails :target:not(h2) {