comment-only fix to tools/update-doxygen.sh

(SkipBuildbotRuns)
NOTREECHECKS=True
NOTRY=True
R=rmistry@google.com
TBR=rmistry

Author: epoger@google.com

Review URL: https://codereview.chromium.org/280703002

git-svn-id: http://skia.googlecode.com/svn/trunk@14703 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/update-doxygen.sh b/tools/update-doxygen.sh
index 87fee32..1f20b4a 100755
--- a/tools/update-doxygen.sh
+++ b/tools/update-doxygen.sh
@@ -12,6 +12,9 @@
 #  export DOXYGEN_TEMPDIR=/tmp/doxygen
 #  export DOXYGEN_COMMIT=false
 #  bash update-doxygen.sh
+#
+# To install doxygen on most Linux systems, you can run:
+# sudo apt-get install doxygen
 
 function check_out_docs {
   svn checkout https://skia-autogen.googlecode.com/svn/docs  # writeable