Update README.md

Warning: use of 'dotnet new --install' is deprecated. Use 'dotnet new install' instead.
For more information, run:
   dotnet new install -h

Author: rmamet

README.md

@@ -13,7 +13,7 @@ Prerequisites:
 Installation:
 
 ```shell
-dotnet new --install CelesteMod.Templates
+dotnet new install CelesteMod.Templates
 ````
 
 From your Celeste/Mods directory:
@@ -41,4 +41,4 @@ These can specified in the following way:
 ```shell
 # Creates a new mod with sample entities, a GitHub build action, and a MonoMod export class 
 dotnet new celestemod --Samples --GitHub --Exports
-```
+```