patch 8.2.0017: OS/2 and MS-DOS are still mentioned

Problem:    OS/2 and MS-DOS are still mentioned, even though support was
            removed long ago.
Solution:   Update documentation. (Yegappan Lakshmanan, closes #5368)
diff --git a/runtime/doc/usr_01.txt b/runtime/doc/usr_01.txt
index cf0c867..23eb304 100644
--- a/runtime/doc/usr_01.txt
+++ b/runtime/doc/usr_01.txt
@@ -71,7 +71,7 @@
 
 Unix: >
 	:!cp -i $VIMRUNTIME/vimrc_example.vim ~/.vimrc
-MS-DOS, MS-Windows, OS/2: >
+MS-Windows: >
 	:!copy $VIMRUNTIME/vimrc_example.vim $VIM/_vimrc
 Amiga: >
 	:!copy $VIMRUNTIME/vimrc_example.vim $VIM/.vimrc