Update README

Replace git URL with HTTPS URL because the latter does not require permission to clone

Signed-off-by: Trung Le <trung.le@ruby-journal.com>
diff --git a/README.md b/README.md
index 45a3df3..395f87c 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@
 To build an image for a Palmetto system:
 
 ```
-git clone --recursive git@github.com:open-power/op-build.git
+git clone --recursive https://github.com/open-power/op-build.git
 cd op-build
 ./op-build palmetto_defconfig && ./op-build
 ```