fix: repair broken code fences in assistant hooks page (#1018)

* feat: document call.timeElapsed hook for assistant hooks

The call.timeElapsed hook exists in the codebase but was missing from
documentation. This adds full documentation including basic usage,
call discipline patterns with graceful wrap-up, and system message
injection for guiding LLM behavior at specific time points.

* fix: repair broken code fences in assistant hooks page

Remove stray four-backtick fence and fix mismatched closing fence
in the low confidence transcripts section, which prevented the
"Multiple hooks for different ranges" heading and code block from
rendering.

Author: dhruva-reddy

fern/assistants/assistant-hooks.mdx

@@ -483,8 +483,6 @@ For example, if your transcriber has a `confidenceThreshold` of 0.4:
 - Transcripts with confidence 0.2-0.4: Hook triggers
 - Transcripts with confidence < 0.2: Discarded
 
-````
-
 ### Multiple hooks for different ranges
 
 You can configure multiple hooks to handle different confidence ranges with different actions:
@@ -512,7 +510,7 @@ You can configure multiple hooks to handle different confidence ranges with diff
     }
   ]
 }
-````
+```
 
 <Note>
 The `assistant.transcriber.endpointedSpeechLowConfidence` hook supports these options: