docs: use single quotes for strings in SQL restrictions

PostgreSQL interprets double quotes as identifier references, not string
literals. Changed all examples to use the portable pattern:
  Table & "field = 'value'"

instead of the MySQL-only pattern:
  Table & 'field = "value"'

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/schemas.py

@@ -899,7 +899,7 @@ def virtual_schema(
     --------
     >>> lab = dj.virtual_schema('my_lab')
     >>> lab.Subject.fetch()
-    >>> lab.Session & 'subject_id="M001"'
+    >>> lab.Session & "subject_id='M001'"
 
     See Also
     --------