Fix "undefined method" error when connecting Stripe

[annemirasol]

Fixes #3357

Changes proposed in this Pull Request:

In #3357, we abstracted Stripe settings handling, and added new getter/setter methods in includes/class-wc-stripe-helper.php. This PR fixes a line that uses the incorrect (non-existent) method name for updating Stripe settings, to fix an "undefined method" error encountered when connecting a Stripe account.

Testing instructions

Code review: verify that the update_main_stripe_settings method exists (in includes/class-wc-stripe-helper.php).

1. On a fresh install, go to WooCommerce > Settings > Payments > Stripe (Finish set up).
2. Connect your test Stripe account by clicking on Connect or create Stripe account, choosing the test account, and clicking Connect.
3. Verify that the connection process finishes, instead of getting an "undefined method" error page (see screen capture). You should get redirected to the Stripe settings page, with Stripe enabled, on success.

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• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[wjrosa]

Good job on your first change! Thanks for fixing this and for addressing the issues I mentioned over Slack. All good 👍