[fix] Fix typo

Fix typo in error message introduced by #2325.

Author: martincostello

src/Microsoft.OpenApi/Models/OpenApiOperation.cs

@@ -365,7 +365,7 @@ public void SerializeAsV2(IOpenApiWriter writer)
                 {
                     if (tag.Target is null)
                     {
-                        throw new OpenApiException($"The OpenAPI tag reference '{tag.Reference.Id}' does reference a valid tag.");
+                        throw new OpenApiException($"The OpenAPI tag reference '{tag.Reference.Id}' does not reference a valid tag.");
                     }
                 }
             }

test/Microsoft.OpenApi.Tests/Models/OpenApiOperationTests.cs

@@ -887,7 +887,7 @@ public async Task SerializeAsJsonAsyncThrowsIfTagReferenceIsUnresolved(OpenApiSp
             };
 
             var exception = await Assert.ThrowsAsync<OpenApiException>(() => operation.SerializeAsJsonAsync(version));
-            Assert.Equal("The OpenAPI tag reference 'two' does reference a valid tag.", exception.Message);
+            Assert.Equal("The OpenAPI tag reference 'two' does not reference a valid tag.", exception.Message);
         }
     }
 }