Fix typo

Change 'This' -> 'These'.

Change-Id: If2f8f92d3adbb8fe4556e5c3ec53e4da31d02d49
diff --git a/doc/source/plugins.rst b/doc/source/plugins.rst
index 803dd08..fda601b 100644
--- a/doc/source/plugins.rst
+++ b/doc/source/plugins.rst
@@ -178,7 +178,7 @@
 ===============
 
 Devstack provides a framework for getting packages installed at an early
-phase of its execution. This packages may be defined in a plugin as files
+phase of its execution. These packages may be defined in a plugin as files
 that contain new-line separated lists of packages required by the plugin
 
 Supported packaging systems include apt and yum across multiple distributions.