Fix tempest plugin cookicutter repo path

cookicutter expect ether git+<repo-url> or it has the
'git' in url, otherwise it give error
- http://paste.openstack.org/show/795952/

plugin cookicutter README is fine but tempest doc does
not mention about it and end up having error
- https://opendev.org/openstack/tempest-plugin-cookiecutter

Change-Id: Ifb12011ac901be57d49a6ae623a24a916837c07d
diff --git a/doc/source/plugins/plugin.rst b/doc/source/plugins/plugin.rst
index a9e2059..ab1b0b1 100644
--- a/doc/source/plugins/plugin.rst
+++ b/doc/source/plugins/plugin.rst
@@ -43,7 +43,7 @@
 In order to create the basic structure with base classes and test directories
 you can use the tempest-plugin-cookiecutter project::
 
-  > pip install -U cookiecutter && cookiecutter https://opendev.org/openstack/tempest-plugin-cookiecutter
+  > pip install -U cookiecutter && cookiecutter https://opendev.org/openstack/tempest-plugin-cookiecutter.git
 
   Cloning into 'tempest-plugin-cookiecutter'...
   remote: Counting objects: 17, done.