buildman: invalid reference to README

The readme file for buildman is called buildman.rst.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/toolchain.py b/tools/buildman/toolchain.py
index 38b0dea..ea1ad1b 100644
--- a/tools/buildman/toolchain.py
+++ b/tools/buildman/toolchain.py
@@ -265,7 +265,7 @@
             print(("Warning: No tool chains. Please run 'buildman "
                    "--fetch-arch all' to download all available toolchains, or "
                    "add a [toolchain] section to your buildman config file "
-                   "%s. See README for details" %
+                   "%s. See buildman.rst for details" %
                    bsettings.config_fname))
 
         paths = []