docs: Change context_schema from ContextSchema to Context (#1156)

## Overview
Fix: use correct Context dataclass instead of ContextSchema in
create_agent

## Type of change

**Type:** [Update existing documentation / Fix typo/bug]

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Co-authored-by: Lauren Hirata Singh <lauren@langchain.dev>

Author: mcavdar

src/oss/python/migrate/langchain-v1.mdx

@@ -690,7 +690,7 @@ class Context:
 agent = create_agent(
     model=model,
     tools=tools,
-    context_schema=ContextSchema  # [!code highlight]
+    context_schema=Context  # [!code highlight]
 )
 
 result = agent.invoke(