Docs: Remove references to elasticsearch directory in plugins

This directory was removed from plugins in elastic#28589, but docs still
referenced it. This commit cleans up the plugin author docs to no longer
refer to it.

docs/plugins/authors.asciidoc

@@ -20,15 +20,10 @@ These examples provide the bare bones needed to get started.  For more
 information about how to write a plugin, we recommend looking at the plugins
 listed in this documentation for inspiration.
 
-[float]
-=== Plugin Structure
-
-All plugin files must be contained in a directory called `elasticsearch`.
-
 [float]
 === Plugin descriptor file
 
-All plugins must contain a file called `plugin-descriptor.properties` in the folder named `elasticsearch`.
+All plugins must contain a file called `plugin-descriptor.properties`.
 The format for this file is described in detail in this example:
 
 ["source","properties",subs="attributes"]
@@ -63,7 +58,7 @@ of nonnegative decimal integers separated by "."'s and may have leading zeros.
 
 |=======================================================================
 
-Note that only jar files in the 'elasticsearch' directory are added to the classpath for the plugin!
+Note that only jar files at the root of the plugin are added to the classpath for the plugin!
 If you need other resources, package them into a resources jar.
 
 [IMPORTANT]