docs(api): fix malformed docstring in session.py

Pre-existing RST typo: ``return_sampling_results`` was missing its
opening double-backtick, leaving a dangling backtick after the
bulk normalisation.  Correct the full inline-code span.

Assisted-by: Claude Code
Signed-off-by: Nigel Jones <nigel_jones@uk.ibm.com>

Author: planetf1

mellea/stdlib/session.py

@@ -926,7 +926,7 @@ async def ainstruct(
 
         Returns:
             A `ComputedModelOutputThunk` if `strategy` is `None` and `await_results` is `False`,
-            else returns a `ModelOutputThunk` if return_sampling_results` is `False``,
+            else returns a `ModelOutputThunk` if `return_sampling_results` is `False`,
             else a `SamplingResult`.
         """
         r = await mfuncs.ainstruct(