fix: change ddg to DDGS (#6480)

This commit updates the duckduckgo search utility by using a more
accurate name in the import statement.

Author: Undertone0809

langchain/utilities/duckduckgo_search.py

@@ -30,7 +30,7 @@ class Config:
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that python package exists in environment."""
         try:
-            from duckduckgo_search import ddg  # noqa: F401
+            from duckduckgo_search import DDGS  # noqa: F401
         except ImportError:
             raise ValueError(
                 "Could not import duckduckgo-search python package. "