Rename __standalone_mode__ to STANDALONE_MODE

Change-Id: I06d5d2690f5a01cd0e4ccd77a044d21d5788ac1d

BUCK

diff --git a/BUCK b/BUCK
index 99a8eca..cccde03 100644
--- a/BUCK
+++ b/BUCK
@@ -2,7 +2,7 @@
 
 MODULE = 'com.googlesource.gerrit.plugins.xdocs.XDocs'
 
-ASCIIDOCTOR = '//lib/asciidoctor:asciidoc_lib' if __standalone_mode__ \
+ASCIIDOCTOR = '//lib/asciidoctor:asciidoc_lib' if STANDALONE_MODE \
   else '//plugins/x-docs/lib/asciidoctor:asciidoc_lib'
 
 gerrit_plugin(