SamdSercom tutorial has a bug: integer is read but only byte is sent

In the "Adding more Serial Interfaces to SAMD microcontrollers (SERCOM)" tutorial, in the section for "Create a new Serial instance", the example code shows:

````
int sensorValue = analogRead(A0);
// print out the value you read on mySerial wired in loopback:
mySerial.write(sensorValue);
````
analogRead() will return values greater than 256 (assuming the resolution of the ADC is the default 10 bits) but the write() will only write a byte. This should be explained, and optionally scaled to fit in a byte.

Start with the “Adding more Serial Interfaces to SAMD microcontrollers (SERCOM)” tutorial and its “Create a new Serial instance” section. Review the analogRead() and mySerial.write() example, then update the explanation so the ADC value’s size and the transmitted byte behavior are clear; done means the example accurately documents the result and any scaling choice.

Written by the indexing model from the issue text.