Fix typos in comments (#19)

proof/proof.go

@@ -67,7 +67,7 @@ func Consistency(size1, size2 uint64) (Nodes, error) {
 	level := uint(bits.TrailingZeros64(size1))
 	index := (size1 - 1) >> level
 	// The consistency proof consists of this node (except if size1 is a power of
-	// two, in which case adding this node would be redundant becase the client
+	// two, in which case adding this node would be redundant because the client
 	// is assumed to know it from a checkpoint), and nodes of the inclusion proof
 	// into this node in the tree of size2.
 	p := nodes(index, level, size2)

proof/proof_test.go

@@ -282,7 +282,7 @@ func TestEphem(t *testing.T) {
 		{index: 13, size: 14, want: id(1, 7)},
 
 		// There is only one node (level 0, index 1024) in the right subtree, but
-		// the ephemeral node is at level 10 rather then level 0. This is because
+		// the ephemeral node is at level 10 rather than level 0. This is because
 		// for the purposes of the proof this node is *effectively* at level 10.
 		{index: 123, size: 1025, want: id(10, 1)},