docs: fix api-ref method-heading naming typos and duplicate section

[jacalata]

Motivation

Auditing api-ref.md against source for the Sphinx migration (#1832)
surfaced 20 headings that reference endpoint methods by the singular
form rather than the plural form actually bound on Server. These have
been drifting since well before Sphinx work started -- users Ctrl-F'ing
datasources.update_connection don't find the section today because
the heading spells it datasource.update_connection. The same audit
turned up a duplicate webhooks.delete section (h3 and h4 both
describing the same method); folded here since it's discovered along
the way.

Behavior change

Docs only. Two commits:

Rename typo'd method headings. Only heading text and the code-sample
invocation line change; section prose is untouched.

Old heading	Correct name	Headings renamed
datasource.*	datasources.*	2
subscription.*	subscriptions.*	4
webhook.*	webhooks.*	6 (delete had two headings)
groupsets.*	group_sets.* (snake_case, matching custom_views, flow_runs)	8

Total: 20 headings renamed.

Remove duplicate webhooks.delete section. Two adjacent sections
described the same method (h3 at line 9573, h4 at line 9684). Kept the
h4 entry -- it lives with the other webhook methods at h4 level, matches
the surrounding template (includes Version info, standard REST API
link, concise Example).

Anchor slug impact. Kramdown regenerates anchor slugs from
headings, so the fragment URLs change (e.g.
#datasourceupdate_connection -> #datasourcesupdate_connection).

Searched across Salesforce-hosted repos for external references to the
old anchor slugs: none found. Existing external links to
tableau.github.io/server-client-python/docs/api-ref all target
section-level anchors (#workbooks, #data-sources, #jobs,
#server, #tableauauth-class) that this PR does not touch.

Internal anchor links in api-ref.md itself: grepped for references to
the renamed method-heading slugs; none found.

Test plan

• Re-ran the audit script against the patched file:
  references_missing bucket went 20 -> 0; the 20 entries redistributed
  into reproducible (+9) and needs_docstring (+11), matching
  source-side docstring coverage.
• Grepped api-ref.md for internal anchor references to the affected
  singular forms; none found.
• Grepped Salesforce-hosted repos for external references to the
  renamed anchor slugs; none found.
• Preview the rendered api-ref.md on the gh-pages Jekyll site;
  confirm the renamed sections render and their TOC entries regenerate.

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