Fix admonition block icon
Fix admonition block icon

The ".icon" CSS selector is applied on the rendered asciidoctor content,
which means a image background with the spring icons is used for the
admonition names.

This commit applies a "background: none" rule on that asciidoctor
content.

martinlippert/sagan,martinlippert/sagan,martinlippert/sagan,spring-io/sagan,martinlippert/sagan,dsyer/sagan,dsyer/sagan,dsyer/sagan,spring-io/sagan,spring-io/sagan,dsyer/sagan,spring-io/sagan