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_05.txt b/runtime/doc/usr_05.txt
index dcef31a..3b43f7e 100644
--- a/runtime/doc/usr_05.txt
+++ b/runtime/doc/usr_05.txt
@@ -472,7 +472,7 @@
 
 	system		plugin directory ~
 	Unix		~/.vim/plugin/
-	PC and OS/2	$HOME/vimfiles/plugin or $VIM/vimfiles/plugin
+	PC		$HOME/vimfiles/plugin or $VIM/vimfiles/plugin
 	Amiga		s:vimfiles/plugin
 	Macintosh	$VIM:vimfiles:plugin
 	Mac OS X	~/.vim/plugin/