docs: ensure L2_FORK_URL points to base sepolia (#276)

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**Motivation:**
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problem you're trying to solve.
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As a devkit user, I want the comments in my .env.example to match the
intention of the variable

**Modifications:**
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- updates `L2_FORK_URL` comment to state that it should point to base
sepolia

Author: grezle

config/.env.example

@@ -3,5 +3,5 @@
 # Ethereum sepolia fork URL used for AVS devnet
 L1_FORK_URL=<RPC URL>
 
-# Ethereum sepolia fork URL used for AVS devnet
+# Ethereum base sepolia fork URL used for AVS devnet
 L2_FORK_URL=<RPC URL>