Remove "chin" in admonition blocks of reference documentation

Originally contributed by @olivergierke via
spring-projects/spring-framework#1520. This commit is based on
the spring.css generated from the SASS original source, see
bclozel/asciidoctor-stylesheet-factory#1.

src/docs/asciidoc/stylesheets/spring.css

@@ -688,4 +688,5 @@ b.conum * { color: inherit !important; }
 
 #toc a:hover { text-decoration: underline; }
 
+.admonitionblock > table { margin-bottom: 0; }
 .admonitionblock > table td.content { border-left: none; }