Fix typo in comment

[bshramin]

[Roasbeef]

Weird panic in itest, re running:

--- FAIL: TestSphinxNodeRelpaySameBatch (0.01s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x5e1064]
goroutine 15 [running]:
testing.tRunner.func1.2({0x6147c0, 0x8d9f60})
	/home/travis/.gimme/versions/go1.17.13.linux.amd64/src/testing/testing.go:1209 +0x24e
testing.tRunner.func1()
	/home/travis/.gimme/versions/go1.17.13.linux.amd64/src/testing/testing.go:1212 +0x218
panic({0x6147c0, 0x8d9f60})
	/home/travis/.gimme/versions/go1.17.13.linux.amd64/src/runtime/panic.go:1038 +0x215
github.com/lightningnetwork/lightning-onion.(*Tx).ProcessOnionPacket(0xc000365f50, 0x0, 0xc00035db80, {0x0, 0x0, 0x0}, 0x1, {0xc0000a9f28, 0x1, 0x1})
	/home/travis/gopath/src/github.com/lightningnetwork/lightning-onion/sphinx.go:787 +0xa4
github.com/lightningnetwork/lightning-onion.TestSphinxNodeRelpaySameBatch(0xc0001f3380)
	/home/travis/gopath/src/github.com/lightningnetwork/lightning-onion/sphinx_test.go:306 +0x35e
testing.tRunner(0xc0001f3380, 0x722990)
	/home/travis/.gimme/versions/go1.17.13.linux.amd64/src/testing/testing.go:1259 +0x102
created by testing.(*T).Run
	/home/travis/.gimme/versions/go1.17.13.linux.amd64/src/testing/testing.go:1306 +0x35a
exit status 2
FAIL	github.com/lightningnetwork/lightning-onion	0.060s