Fixed Some Problems from Demo Examples

[seanzhangkx8]

Description

Main changes throughout examples:

1. replace occurrences of User (from older version) with Speaker (from new versions)
2. modify minor errors within notebooks(replace utt id non exists, reference error, etc.)

Motivation and Context

To provide updated and correct examples for users to better understand how to use convokit.

How has this been tested?

Run in Jupyter notebook using newest version of ConvoKit (from GitHub main branch) with python version 3.9.0, following all contribution guidelines.

[calebchiam]

Nice work!