docs(planning): align Bulkhead spec with implementation (TimeoutError, not asyncio.TimeoutError)

In Python 3.11+, asyncio.TimeoutError IS builtins.TimeoutError. The
implementation uses the bare name; align the spec snippet so spec and
code agree. Cosmetic — no behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lesnik512

planning/specs/2026-06-05-bulkhead-design.md

@@ -109,7 +109,7 @@ async def __call__(self, request, next):
         else:
             async with asyncio.timeout(self._acquire_timeout):
                 await self._sem.acquire()
-    except asyncio.TimeoutError as exc:
+    except TimeoutError as exc:  # builtins.TimeoutError, which `asyncio.timeout` raises in 3.11+
         raise BulkheadFullError(
             max_concurrent=self._max_concurrent,
             acquire_timeout=self._acquire_timeout,