Fix example of node.type [ci skip]

```
% docker run -it --rm rubylang/all-ruby env ALL_RUBY_SINCE=ruby-2.6 ./all-ruby -e 'root = RubyVM::AbstractSyntaxTree.parse("x = 1 + 2")
p [root, root.type]
call = root.children[2]
p [call, call.type]
'
ruby-2.6.0          [#<RubyVM::AbstractSyntaxTree::Node:SCOPE@1:0-1:9>, :SCOPE]
                    [#<RubyVM::AbstractSyntaxTree::Node:LASGN@1:0-1:9>, :LASGN]
...
ruby-2.7.0          [#<RubyVM::AbstractSyntaxTree::Node:SCOPE@1:0-1:9>, :SCOPE]
                    [#<RubyVM::AbstractSyntaxTree::Node:LASGN@1:0-1:9>, :LASGN]
```

1 files changed, 1 insertions, 1 deletions

diff --git a/ast.rb b/ast.rb
index 1fedf76649..d7e9df8678 100644
--- a/ast.rb
+++ b/ast.rb
@@ -84,7 +84,7 @@ class RubyVM
       #    root = RubyVM::AbstractSyntaxTree.parse("x = 1 + 2")
       #    root.type # => :SCOPE
       #    call = root.children[2]
-      #    call.type # => :OPCALL
+      #    call.type # => :LASGN
       def type
         __builtin_ast_node_type
       end