Correct quoting in command used to set platform.txt version value

Single quotes caused what was intended to be a reference to the VERSION variable to instead be a string, resulting in the value of the platform.txt version property being set to "$VERSION":

version=$VERSION

package.sh

@@ -64,7 +64,7 @@ mv _bootloaders bootloaders
 
 #Patch title in platform.txt
 sed -i "s/Arduino Mbed OS Boards/Arduino Mbed OS ${FLAVOUR^} Boards/g" platform.txt
-sed -i 's/9.9.9/$VERSION/g' platform.txt
+sed -i "s/9.9.9/$VERSION/g" platform.txt
 
 BASE_FOLDER=`basename $PWD`
 
@@ -77,4 +77,4 @@ echo FILENAME=ArduinoCore-mbed-$VERSION.tar.bz2 > /tmp/env
 else
 echo FILENAME=ArduinoCore-mbed-$FLAVOUR-$VERSION.tar.bz2 > /tmp/env
 fi
-cd -
+cd -