Changed configuration of Lazy to make it threadsafe

Author: darrelmiller

src/Microsoft.OpenApi/Validations/ValidationRuleSet.cs

@@ -30,7 +30,7 @@ public static ValidationRuleSet DefaultRuleSet
             {
                 if (_defaultRuleSet == null)
                 {
-                    _defaultRuleSet = new Lazy<ValidationRuleSet>(() => BuildDefaultRuleSet(), isThreadSafe: false).Value;
+                    _defaultRuleSet = new Lazy<ValidationRuleSet>(() => BuildDefaultRuleSet(), isThreadSafe: true).Value;
                 }
 
                 return _defaultRuleSet;