Fixed error code for MultiPartFileMissing (#7148)

Author: glen-84

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/__snapshots__/HttpMultipartMiddlewareTests.MissingFile_Test.snap

@@ -6,7 +6,7 @@
     {
       "message": "File of key '1' is missing.",
       "extensions": {
-        "code": "HC00038"
+        "code": "HC0038"
       }
     }
   ],

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/__snapshots__/HttpMultipartMiddlewareTests.MissingKeyInMap_Test.snap

@@ -6,7 +6,7 @@
     {
       "message": "File of key '' is missing.",
       "extensions": {
-        "code": "HC00038"
+        "code": "HC0038"
       }
     }
   ],

src/HotChocolate/Core/src/Abstractions/ErrorCodes.cs

@@ -135,7 +135,7 @@ public static class Server
         /// <summary>
         /// A key is referring to a file that was not provided.
         /// </summary>
-        public const string MultiPartFileMissing = "HC00038";
+        public const string MultiPartFileMissing = "HC0038";
 
         /// <summary>
         /// The variable path is referring to a variable that does not exist.