docs: fix errors in multiple permissive policies example

[nielskaspers]

Summary

Fix multiple documentation errors in the 0006 multiple permissive policies lint example that made the "How to Resolve" section incorrect and misleading.

Issue

Fixes supabase/supabase#44600

Changes

• Fix typo: "departemnt" → "department"
• Fix Policy B description: "at or above a certain grade level" → "at or below the querying user's grade level" to match the <= operator in the code
• Align stated intention ("below") with the <= operator ("at or below")
• Fix consolidated policy: change or to and so both conditions must be satisfied (matching the intended behavior)
• Fix consolidated policy operator: >= → <= to match the original Policy B
• Add clarity: specify "regardless of grade level" and "regardless of department" to better explain the permissive OR behavior

Testing

• Read through the corrected example to verify the text, code, and stated intention are all consistent
• The consolidated policy now correctly requires both same-department AND at-or-below grade level, which matches the stated intention