Update OpenApiConstants.cs

Co-authored-by: Vincent Biret <vincentbiret@hotmail.com>

Author: mdaneri

src/Microsoft.OpenApi/Models/OpenApiConstants.cs

@@ -138,7 +138,7 @@ public static class OpenApiConstants
         /// <summary>
         /// Extension: x-jsonSchema-patternProperties
         /// </summary>
-        public const string PatternPropertiesExtension = "x-jsonSchema-patternProperties";
+        public const string PatternPropertiesExtension = "x-jsonschema-patternProperties";
 
         /// <summary>
         /// Field: Version