Update 31-to-50.md (#19961)

Author: pranavkm

aspnetcore/migration/31-to-50.md

@@ -75,7 +75,7 @@ For Blazor WebAssembly projects, apply the following changes in the project file
 1. Update the following properties:
 
     ```diff
-    <Project Sdk="Microsoft.NET.Sdk.Web">
+    <Project Sdk="Microsoft.NET.Sdk.BlazorWebAssembly">
 
       <PropertyGroup>
     -     <TargetFramework>netstandard2.1</TargetFramework>