Fix grammar in docs/README.md (#6303)

Author: ctyar

docs/README.md

@@ -2,4 +2,4 @@ Contributor documentation
 =========================
 
 The primary audience for documentation in this folder is contributors to ASP.NET Core.
-If you are looking for documentation on to *use* ASP.NET Core, go to <https://docs.asp.net>.
+If you are looking for documentation on how to *use* ASP.NET Core, go to <https://docs.asp.net>.