Fix doc comment typo in JsonSerializerOptions.cs (#48590)

The following sentence in the doc comments for the `IgnoreReadOnlyFields` member:

> A property is read-only if it isn't marked with the <c>readonly</c> keyword.

Should read:

> A field is read-only if it is marked with the <c>readonly</c> keyword.

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.cs

@@ -313,7 +313,7 @@ public bool IgnoreReadOnlyProperties
 
         /// <summary>
         /// Determines whether read-only fields are ignored during serialization.
-        /// A property is read-only if it isn't marked with the <c>readonly</c> keyword.
+        /// A field is read-only if it is marked with the <c>readonly</c> keyword.
         /// The default value is false.
         /// </summary>
         /// <remarks>