Fix flake8 issues

langchain/chains/fork_with_follow_ups.py

@@ -8,7 +8,7 @@ from langchain.chains.base import Chain
 
 
 class ForkChain(Chain, BaseModel):
-    """Conditionally executes a follow up chain based on the output of a decision chain."""
+    """Conditionally executes follow up chain based on output of a decision chain."""
 
     decision_chain: LLMChain
     follow_up_chains: Dict[str, Chain]
@@ -21,6 +21,7 @@ class ForkChain(Chain, BaseModel):
 
     @validator("follow_up_chains")
     def default_in_follow_up_chains(cls, v: Dict[str, Chain]) -> Dict[str, Chain]:
+        """Make sure that `default` key exists in follow_up_chains."""
         if "default" not in v:
             raise ValueError(
                 "`follow_up_chains` must contain a 'default' option. "