doc: samples: kernel: add relevant-api entry to msg_queue sample

Ensures this sample is properly cross-referencing the Doxygen API group
it's demonstrating.

Also fix wording of the sample title to align with guidelines

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

Author: kartben

samples/kernel/msg_queue/README.rst

@@ -1,7 +1,8 @@
 .. zephyr:code-sample:: msg_queue
    :name: Message Queue
+   :relevant-api: msgq_apis
 
-   Implements a basic message queue producer/consumer thread pair.
+   Implement a basic message queue producer/consumer thread pair.
 
 Overview
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