tests: fixed resulting binary name in README

We no longer produce nanokernel.elf or microkernel.elf.

Change-Id: I37cb985ddc90f4129f122b9209d306898a71a596
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/tests/benchmark/footprint/nanokernel/README.txt b/tests/benchmark/footprint/nanokernel/README.txt
index 073d3b2..591ab58 100644
--- a/tests/benchmark/footprint/nanokernel/README.txt
+++ b/tests/benchmark/footprint/nanokernel/README.txt
@@ -95,4 +95,4 @@
 Additional notes:
 
 Various host utilities (such as the Unix "size" utility) can be used to
-determine the footprint of the resulting outdir/nanokernel.elf image.
+determine the footprint of the resulting zephyr.elf image.