RPI install instructions minor inconsistency

[yonjah]

This is a...

• [ ] Feature Request
• [x] Bug Report

Problem: https://github.com/thingsboard/thingsboard.github.io/blob/master/docs/user-guide/install/rpi.md#thingsboard-service-installation

# Update ThingsBoard memory usage and restrict it to 150MB in /etc/thingsboard/conf/thingsboard.conf
export JAVA_OPTS="$JAVA_OPTS -Dplatform=rpi -Xms256M -Xmx256M"

Comment asks to allocate 150MB of memory but command allocates 256MB

Proposed Solution: Make sure comment and command are indicating the correct memory value (If I understand the git log it is probably 256MB)

Page to Update: https://github.com/thingsboard/thingsboard.github.io/blob/master/docs/user-guide/install/rpi.md

[xalt7x]

Fixed by #224