Add missing whitespace in config description

Change-Id: Ie7e66b9de0a1d14de795166671d31fc9bbeae206
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 0554082..ea9c528 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -427,7 +427,7 @@
 # (string value)
 #alt_password=<None>
 
-# Administrative Username to use forKeystone API requests.
+# Administrative Username to use for Keystone API requests.
 # (string value)
 #admin_username=admin