(Core) Small Change in Docstring for method partial for BasePromptTemplate (#28969)

- **Description:** Very small change in Docstring for
`BasePromptTemplate`
- **Issue:** #28966

Author: keenborder786

libs/core/langchain_core/prompts/base.py

@@ -264,7 +264,7 @@ def partial(self, **kwargs: Union[str, Callable[[], str]]) -> BasePromptTemplate
         """Return a partial of the prompt template.
 
         Args:
-            kwargs: Union[str, Callable[[], str], partial variables to set.
+            kwargs: Union[str, Callable[[], str]], partial variables to set.
 
         Returns:
             BasePromptTemplate: A partial of the prompt template.