patch 9.0.1653: Amiga: default 'viewdir' may not work

Problem:    Amiga: default 'viewdir' may not work.
Solution:   Use "home:" instead of "$VIM". Add a test. (Christian Brabandt,
            closes #12576)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 6653807..9292b05 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -8912,9 +8912,9 @@
 	displayed when 'verbosefile' is set.
 
 						*'viewdir'* *'vdir'*
-'viewdir' 'vdir'	string	(default for Amiga "$VIM/vimfiles/view",
+'viewdir' 'vdir'	string	(default for Amiga "home:vimfiles/view",
 					 for Win32: "$HOME/vimfiles/view",
-					 for Unix: "~/.vim/view",
+					 for Unix: "$HOME/.vim/view",
 					 for macOS: "$VIM/vimfiles/view"
 					 for VMS: "sys$login:vimfiles/view")
 			global