experimental[patch]: fixed namespace bug (#13585)

It was :
`from langchain.schema.prompts import BasePromptTemplate`
but because of the breaking change in the ns, it is now
`from langchain.schema.prompt_template import BasePromptTemplate`

This bug prevents building the API Reference for the langchain_experimental

libs/experimental/langchain_experimental/prompts/load.py

@@ -6,7 +6,7 @@ from typing import Union
 
 import yaml
 from langchain.prompts.loading import load_prompt_from_config, try_load_from_hub
-from langchain.schema.prompts import BasePromptTemplate
+from langchain_core.prompts import BasePromptTemplate
 
 
 def load_prompt(path: Union[str, Path]) -> BasePromptTemplate: