Added Amazon link for LiPo battery

Fix validation steps computation

Screw location for Pi cam can result in short

added sava instock site link

Due to screw head size and placement, using top screw holes on Pi cam can cause a short. Using bottom two holes avoids this potential issue.

Reported by @Ezward on the Donkey Car Slack: https://donkeycar.slack.com/archives/C487786DC/p1522638679000068?thread_ts=1522442985.000360&cid=C487786DC

Clarification on how to begin calibration

No underscores in host name

I bought mine on Amazon so I put the link in there as an option.

I tried to run calibration by going into the donkey environment on my Mac instead of ssh-ing into my Pi. Hopefully adding this line will enable others to avoid my mistake and wasting time.

Update instructions - no hyphens in host name

Using underscore in the host name prevents you from pinging it via host name. Updating instructions to indicate no underscores.

According to RFC 1123, section 2.1 "Host Names and Numbers" host names are limited to just letters, digits, and hyphens.

Modify error parameters

Modify system settings