Memory type error in slides/02-Using-gem5/01-stdlib.md

[Shaan106]

The imports and use of memory are not coherent in the markdown notes.

import:
from gem5.components.memory.single_channel import SingleChannelDDR4_2400

usage:
memory = SingleChannelDDR3_1600()

However, the finished example uses
memory = SingleChannelDDR4_2400()