board/bsh/imx8mn-bsh-smm-s2/readme.txt: fix typo

It was a leftover from the copy-and-paste of the readme.txt file of its
almost twin sister BSH SMM S2 PRO.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/bsh/imx8mn-bsh-smm-s2/readme.txt

@@ -2,7 +2,7 @@ i.MX8MN BSH SMM S2
 ==================
 
 This tutorial describes how to use the predefined Buildroot
-configuration for the i.MX8MN BSH SMM S2 PRO board.
+configuration for the i.MX8MN BSH SMM S2 board.
 
 Building
 --------