rubric20-semantic (#154): fix introduced typos and trailing whitespace

[realmarcin]

Context

Minor cleanup from review of #154 (rubric-review branch). Trivial, but should be fixed before merge.

Typos introduced in d4d-rubric20-semantic.md

1. Q5 (Data File Size Availability): "Note that sample size only enables and estimate of the file size." → an
2. Q4 (File Enumeration and Type Variety): "Variety can indicate multi-modal data if indicated description, purposes, or keywords." → missing "in" ("if indicated in description, …").

Trailing whitespace introduced

Several lines gained trailing whitespace (lint noise):

• The intro sentence ending "…and semantic correctness. "
• Q16 (Findability): "Verify presence of persistent URLs. "
• the agent title/header line

Fix

Correct the two typos and strip trailing whitespace.

[realmarcin]

Adversarial review (Codex) verdict: KEEP but DOWNGRADE — real, not merge-blocking.

Typos and trailing whitespace are confirmed real (e.g. .claude/agents/d4d-rubric20-semantic.md:16, :161) but are cleanup, not a blocker. Fold into whichever PR fixes the substantive items (#155, #161). Codex also confirms the emoji removal and example-wording changes are immaterial — no action needed beyond this cleanup.