Bug 1861731 - Fix screenshot_on_failure typo in Talos. r=perftest-reviewers,sparky

This patch fixes the logic for --screenshot-on-failure in Talos that was
caught.

Differential Revision: https://phabricator.services.mozilla.com/D202987

testing/mozharness/mozharness/mozilla/testing/talos.py

@@ -577,7 +577,7 @@ class Talos(
         if self.config.get("code_coverage", False):
             options.extend(["--code-coverage"])
         if (
-            self.config.get("--screenshot-on-failure", False)
+            self.config.get("screenshot_on_failure", False)
             or os.environ.get("MOZ_AUTOMATION", None) is not None
         ):
             options.extend(["--screenshot-on-failure"])