Update OpenApiJsonWriter.cs

Author: SimonCropp

src/Microsoft.OpenApi/Writers/OpenApiJsonWriter.cs

@@ -42,7 +42,7 @@ public OpenApiJsonWriter(TextWriter textWriter, OpenApiWriterSettings settings,
         /// <summary>
         /// Indicates whether or not the produced document will be written in a compact or pretty fashion.
         /// </summary>
-        private bool _produceTerseOutput;
+        private readonly bool _produceTerseOutput;
 
         /// <summary>
         /// Base Indentation Level.