update doc for yaml format for null

I found that ~ is used for null instead of -

Author: ananda-agrawal

components/yaml/yaml_format.rst

@@ -114,7 +114,7 @@ Numbers
 Nulls
 ~~~~~
 
-Nulls in YAML can be expressed with ``null`` or ``-``.
+Nulls in YAML can be expressed with ``null`` or ``~``.
 
 Booleans
 ~~~~~~~~
@@ -268,4 +268,4 @@ Comments can be added in YAML by prefixing them with a hash mark (``#``):
     Comments are simply ignored by the YAML parser and do not need to be
     indented according to the current level of nesting in a collection.
 
-.. _YAML: http://yaml.org/
+.. _YAML: http://yaml.org/