Capitalize 'Nutshell'

Author: Theofilos Manitaras

README.md

@@ -13,7 +13,7 @@
 [![Slack](https://reframe-slack.herokuapp.com/badge.svg)](https://reframe-slack.herokuapp.com/)<br/>
 [![License](https://img.shields.io/badge/License-BSD%203--Clause-blue.svg)](https://opensource.org/licenses/BSD-3-Clause)
 
-# ReFrame in a nutshell
+# ReFrame in a Nutshell
 
 ReFrame is a framework for writing regression tests for HPC systems.
 The goal of this framework is to abstract away the complexity of the interactions with the system, separating the logic of a regression test from the low-level details, which pertain to the system configuration and setup.