doc: Fix case in titles

Unified case in titles.
Fixed small grammar issues.

Signed-off-by: Francesco Servidio <francesco.servidio@nordicsemi.no>
diff --git a/sim/README.rst b/sim/README.rst
index d07bc11..fb9012b 100644
--- a/sim/README.rst
+++ b/sim/README.rst
@@ -1,4 +1,4 @@
-MCUboot Simulator
+MCUboot simulator
 #################
 
 This is a small simulator designed to exercise the mcuboot upgrade