small refinement in the outer/inner doc


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15206 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/README_DEVELOPERS b/README_DEVELOPERS
index 997f495..fdd70b5 100644
--- a/README_DEVELOPERS
+++ b/README_DEVELOPERS
@@ -250,6 +250,7 @@
 
  To compare the performance of multiple Valgrind versions, do :
     perl perf/vg_perf --outer-valgrind=../outer/.../bin/valgrind \
+      --outer-tool=callgrind \
       --vg=../inner_xxxx --vg=../inner_yyyy perf
   (where inner_xxxx and inner_yyyy are the toplevel directories of
   the versions to compare).