Add helptext for snapshotctl map

Test: no need
Change-Id: I709d02ccd478d6bed34808dc20cbba9ed89097c1
diff --git a/fs_mgr/libsnapshot/snapshotctl.cpp b/fs_mgr/libsnapshot/snapshotctl.cpp
index 5eb2003..67189d4 100644
--- a/fs_mgr/libsnapshot/snapshotctl.cpp
+++ b/fs_mgr/libsnapshot/snapshotctl.cpp
@@ -36,7 +36,9 @@
                  "  dump\n"
                  "    Print snapshot states.\n"
                  "  merge\n"
-                 "    Deprecated.\n";
+                 "    Deprecated.\n"
+                 "  map\n"
+                 "    Map all partitions at /dev/block/mapper\n";
     return EX_USAGE;
 }