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/docs/testplan-zephyr.md b/docs/testplan-zephyr.md
index 63b57e6..9b70151 100644
--- a/docs/testplan-zephyr.md
+++ b/docs/testplan-zephyr.md
@@ -1,4 +1,4 @@
-# Zephyr Test Plan
+# Zephyr test plan
 
 The following roughly describes how MCUboot is tested on Zephyr.  The
 testing is done with the code in `samples/zephyr`.  These examples