Fix escaped character error in BOPE tutorial and simplify plotting (p…

…ytorch#1907)

Summary:
## Motivation

* A Pandas upgrade changed the way that Greek letters needed to be escaped for plotting, so in pytorch#1777 we changed "\\\\zeta" to "\\zeta". However, this caused failures for those using an older version of Pandas. Therefore, we will simply represent it as "zeta" for plotting.
* Since only one replication is run, there is no need to compute an "sem", since it is always NaN over one data point. So I removed that part of the analysis and plotting logic.

### Have you read the [Contributing Guidelines on pull requests](https://github.com/pytorch/botorch/blob/main/CONTRIBUTING.md#pull-requests)?

Yes

Pull Request resolved: pytorch#1907

Test Plan:
I ran the notebook.

## Related PRs

pytorch#1777

Reviewed By: Balandat

Differential Revision: D47096998

Pulled By: esantorella

fbshipit-source-id: 8739c2c5685ed76e41258e46316a69dcc5527bc9