Use .category to pick admonition color in terminal

As per the manual, the styling of the admonition should be determined by
its type/category, a lowecase string stored in the .category field.

stdlib/Markdown/src/render/terminal/render.jl

@@ -34,13 +34,13 @@ end
 function term(io::IO, md::Admonition, columns)
  col = :default
  # If the types below are modified, the page manual/documentation.md must be updated accordingly.
- if lowercase(md.title) == "danger"
+ if md.category == "danger"
  col = Base.error_color()
- elseif lowercase(md.title) == "warning"
+ elseif md.category == "warning"
  col = Base.warn_color()
- elseif lowercase(md.title) in ("info", "note")
+ elseif md.category in ("info", "note")
  col = Base.info_color()
- elseif lowercase(md.title) == "tip"
+ elseif md.category == "tip"
  col = :green
  end
  printstyled(io, ' '^margin, "│ "; color=col, bold=true)