Fixed slashes in ////zeta in BOPE tutorial (#1777)

Summary:
## Motivation

A pandas update broke the CI. This fixes it.

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

Yes

Pull Request resolved: #1777

Test Plan: Repro'd the error locally and confirmed that this fixes it. The CI should pass now.

Reviewed By: Balandat

Differential Revision: D44682690

Pulled By: esantorella

fbshipit-source-id: b8601bf1d3b340fe7e407c40bef216562d66b121

tutorials/bope.ipynb

@@ -555,7 +555,7 @@
         "# Prepare PE data for plots\n",
         "within_df = pd.DataFrame(within_session_results)\n",
         "within_df[\"pe_strategy\"] = within_df[\"pe_strategy\"].str.replace(\n",
-        "    \"EUBO-zeta\", r\"$EUBO-\\\\zeta$\"\n",
+        "    \"EUBO-zeta\", r\"$EUBO-\\zeta$\"\n",
         ")\n",
         "within_df = (\n",
         "    within_df.groupby([\"n_comps\", \"pe_strategy\"])\n",