scripts: build_packages: Update reference

Update './build_packages' -> 'build_packages' since we've migrated to
Python.

BUG=b:218331649
TEST=CQ

Change-Id: I43ffe63a72428e6d6553f922c7640e0706cb5d23
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3840707
Auto-Submit: Cindy Lin <xcl@google.com>
Commit-Queue: Lizzy Presland <zland@google.com>
Reviewed-by: Lizzy Presland <zland@google.com>
Tested-by: Cindy Lin <xcl@google.com>
diff --git a/scripts/build_packages.py b/scripts/build_packages.py
index 99945e5..c25a1a4 100644
--- a/scripts/build_packages.py
+++ b/scripts/build_packages.py
@@ -295,7 +295,7 @@
   # In local dev env cases, compiler_proxy is expected to keep running.
   # In such a case;
   #   $ python ${GOMA_DIR}/goma_ctl.py ensure_start
-  #   $ ./build_packages (... and options without --run-goma ...)
+  #   $ build_packages (... and options without --run-goma ...)
   # is an expected commandline sequence. If you set --run-goma flag while
   # compiler_proxy is already running, the existing compiler_proxy will be
   # stopped.