Define a quoting rule for yaml examples OAI#1720.

## This PR

Define a quoting rules for yaml examples.

Author: ioggstream

versions/3.0.2.md

@@ -128,6 +128,8 @@ In order to preserve the ability to round-trip between YAML and JSON formats, YA
 
 - Tags MUST be limited to those allowed by the [JSON Schema ruleset](http://www.yaml.org/spec/1.2/spec.html#id2803231).
 - Keys used in YAML maps MUST be limited to a scalar string, as defined by the [YAML Failsafe schema ruleset](http://yaml.org/spec/1.2/spec.html#id2802346).
+- strings MUST contain no quoting unless necessary and default to double quote unless single quote is necessary.
+
 
 **Note:** While APIs may be defined by OpenAPI documents in either YAML or JSON format, the API request and response bodies and other content are not required to be JSON or YAML.