fix spelling mistake

SimpleHttpServer/HttpServer.cs

@@ -203,7 +203,7 @@ public sealed class HttpServer {
     private static async Task HandleDefaultErrorPageAsync(RequestContext ctx, int errorCode) {
         await ctx.WriteLineToRespAsync($"""
             <body>
-            <h1>Oh no, and error occurred!</h1>
+            <h1>Oh no, an error occurred!</h1>
             <p>Code: {errorCode}</p>
             </body>
             """);