Rename git_host to server in github driver

To be more consistent with the gerrit driver.

Change-Id: I96da4de53294db238dd22aca7387434d86e97a46
diff --git a/doc/source/admin/drivers/github.rst b/doc/source/admin/drivers/github.rst
index 0cbf895..9f116ee 100644
--- a/doc/source/admin/drivers/github.rst
+++ b/doc/source/admin/drivers/github.rst
@@ -41,14 +41,14 @@
   Path to SSH key to use when cloning github repositories.
   ``sshkey=/home/zuul/.ssh/id_rsa``
 
-**git_host**
+**server**
   Optional: Hostname of the github install (such as a GitHub Enterprise)
   If not specified, defaults to ``github.com``
-  ``git_host=github.myenterprise.com``
+  ``server=github.myenterprise.com``
 
 **canonical_hostname**
   The canonical hostname associated with the git repos on the GitHub
-  server.  Defaults to the value of **git_host**.  This is used to
+  server.  Defaults to the value of **server**.  This is used to
   identify repos from this connection by name and in preparing repos
   on the filesystem for use by jobs.  This only needs to be set in the
   case where the canonical public location of the git repos is not the