[DOC] Update description about shared library suffix

Loading an extension library with ".dll" suffix on Windows was very
old behavior, and the suffix must be ".so" since 2004.  See commits
removing DLEXT2 181a3a2 and
b76ad15.

Instead, use macOS as an example, which uses ".bundle".

load.c

@@ -963,7 +963,7 @@ static VALUE rb_require_string_internal(VALUE fname, bool resurrect);
  *  found.  If the file named cannot be found, a LoadError will be raised.
  *
  *  For Ruby extensions the filename given may use ".so" or ".o".  For example,
- *  on Windows the socket extension is "socket.dll" and
+ *  on macOS the socket extension is "socket.bundle" and
  *  <code>require 'socket.so'</code> will load the socket extension.
  *
  *  The absolute path of the loaded file is added to