Fix merkle comment typo (#1393)

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### Correct MerkleTree doc comment in `pkg/merkle/tree.go` to state root
at index 0 and note nil empty nodes and leaves
Updates the `MerkleTree` struct documentation to reflect 0-indexed root
and clarifies that empty nodes and leaves are `nil` and not hashed in
[tree.go](https://github.com/xmtp/xmtpd/pull/1393/files#diff-4a0ec8af27f891e4481136ec6a06b2eea6423d8992b8ea8c5c14d841b05b4d9a).

#### 📍Where to Start
Start with the `MerkleTree` struct doc comment in
[tree.go](https://github.com/xmtp/xmtpd/pull/1393/files#diff-4a0ec8af27f891e4481136ec6a06b2eea6423d8992b8ea8c5c14d841b05b4d9a).

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Author: fbac

pkg/merkle/tree.go

@@ -13,10 +13,10 @@ type Leaf []byte
 
 // MerkleTree is a binary Merkle tree.
 //
-// tree has the collection of nodes, where:
-// - The tree is 1-indexed, so root is at index 1.
-// - The internal nodes are at index 1 to N-1.
-// - The leaves are at index N to 2N-1.
+// The tree is built as a balanced tree, where:
+// The root is at index 0, internal nodes are at index 1 to N-1, and the leaves are at index N to 2N-1.
+// Empty nodes and leaves are nil and not hashed, to save space.
+
 type MerkleTree struct {
 	tree   []Node
 	leaves []Leaf