Fix a capitalization mistake in README.rst

Reviewing the README.rst of tempest,
I noticed a wrong capitalization.
The word "Scenario" is capitalized when
instead it should be lower-case.
So I fix it.

Change-Id: I22208608e0b19df6da9d718e44499a1379ecd011
diff --git a/README.rst b/README.rst
index f8059ae..044ae09 100644
--- a/README.rst
+++ b/README.rst
@@ -15,7 +15,7 @@
 
 This is a set of integration tests to be run against a live OpenStack
 cluster. Tempest has batteries of tests for OpenStack API validation,
-Scenarios, and other specific tests useful in validating an OpenStack
+scenarios, and other specific tests useful in validating an OpenStack
 deployment.
 
 Design Principles