README: point to correct location of README.remus

The said file was moved in 20857ac ("remus: move remus README to docs
directory").

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Yang Hongyang <yanghy@cn.fujitsu.com>
Cc: Shriram Rajagopalan <rshriram@cs.ubc.ca>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Yang Hongyang <yanghy@cn.fujitsu.com>

README

@@ -75,7 +75,7 @@ disabled at compile time:
     * systemd daemon development files
     * Development install of libnl3 (e.g., libnl-3-200,
       libnl-3-dev, etc).  Required if network buffering is desired
-      when using Remus with libxl.  See tools/remus/README for detailed
+      when using Remus with libxl.  See docs/README.remus for detailed
       information.
 
 Second, you need to acquire a suitable kernel for use in domain 0. If