[DOC] Improve CSS for "toggle source" hovering over one more method signatures

This commit improves the behavior of showing the "toggle source" element on mouseover.

For example, when a method has one more signatures by using `:call-seq:`,

```ruby
# :call-seq:
#   foo {|element| ... } -> self
#   foo -> new_enumeration
def foo
end
```

The current CSS doesn't show "toggle source" even when hovering the second signature `foo -> new_enumeration`.
But this change will show "toggle source" always when hovering over any signature.

For details about the `.method-header` element, see `lib/rdoc/generator/template/darkfish/class.rhtml`:
https://github.com/ruby/rdoc/blob/0e060c69f51ec4a877e5cde69b31d47eaeb2a2b9/lib/rdoc/generator/template/darkfish/class.rhtml#L101-L124

For example, see https://docs.ruby-lang.org/en/3.2/Array.html#method-i-delete

Author: ybiquitous

lib/rdoc/generator/template/darkfish/css/rdoc.css

@@ -567,7 +567,7 @@ main .method-click-advice {
   line-height: 20px;
   background: url(../images/zoom.png) no-repeat right top;
 }
-main .method-heading:hover .method-click-advice {
+main .method-header:hover .method-click-advice {
   visibility: visible;
 }