Update msr3-controlling-dc-motor.md

[gniticxe]

changed parameter of analogWrite to match 255 max (was 100%)

What This PR Changes

• Documentation / tutorial for Arduino Motor Shield incorrectly had max output set to 100 instead of 255.

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[CLAassistant]

All committers have signed the CLA.

[AdamRaichu]

Please merge this! Beginners to Arduino may not realize that analogWrite accepts values between 0-255.