Fix asf GitHub sync issue 20181211

Author: William Guo <guoyp@apache.org>

Closes #469 from guoyuepeng/fix_asf_github_sync_issue_20181211.
diff --git a/README.md b/README.md
index adc820d..15d5371 100644
--- a/README.md
+++ b/README.md
@@ -68,7 +68,7 @@
 * Clone down your fork
 * Implement your feature or bug fix and commit changes
 * Push the branch up to your fork
-* Send a pull request to Apache Griffin branch
+* Send a pull request to Apache Griffin master branch
 
 
 ## References