Update 2_plugin_start.md (#29)

Changed .net standard version from 2.0 to 2.1 for games using unity 2021.2 or newer

articles/dev_guide/plugin_tutorial/2_plugin_start.md

@@ -31,7 +31,7 @@ To create a project in the folder, do the following depending on the game type y
 
     You can follow this general-purpose choice process:
 
-    * If the game has `netstandard.dll` in `<Game Name>_Data/Managed` folder, your TFM is `netstandard2.0`. If you run into reference errors, target `net472`.  
+    * If the game has `netstandard.dll` in `<Game Name>_Data/Managed` folder, your TFM is `netstandard2.0` (for Unity 2021.1.x or older) or `netstandard2.1` (for Unity 2021.2 or newer). If you run into reference errors, target `net472`.  
     **OR**  
     * If the game's `mscorlib.dll` file version (right click the file -> `Properties` -> `Details`, `exiftool mscorlib.dll | grep "File Version"` on Linux) is at least `4.0.0.0` or newer, your TFM is `net46`  
     **OR**