ast.rb: reduce nesting

ast.rb

@@ -1,7 +1,5 @@
 # for ast.c
 
-class RubyVM
-
 # AbstractSyntaxTree provides methods to parse Ruby code into
 # abstract syntax trees. The nodes in the tree
 # are instances of RubyVM::AbstractSyntaxTree::Node.
@@ -19,7 +17,7 @@ class RubyVM
 # like to make RubyVM::AbstractSyntaxTree stable, please join the discussion
 # at https://bugs.ruby-lang.org/issues/14844.
 #
-  module AbstractSyntaxTree
+module RubyVM::AbstractSyntaxTree
 
   #  call-seq:
   #     RubyVM::AbstractSyntaxTree.parse(string) -> RubyVM::AbstractSyntaxTree::Node
@@ -143,4 +141,3 @@ class RubyVM
     end
   end
 end
-end