Replace 'master' with 'main' in various files (#71)

* Replace 'master' with 'main' in various files

* Update README.md

samples/aspnetcore/security/README.md

@@ -1,3 +1,3 @@
 # Security samples
 
-Please find additional security and authentication related samples here: https://github.com/aspnet/AspNetCore/tree/master/src/AuthSamples/samples
+Please find additional security and authentication related samples here: https://github.com/dotnet/aspnetcore/tree/main/src/Security/samples