fixed mixed up comments

Discovered this while working on #1838.

In the following scenario, at reconnection:
- `localCommit.index = 7`
- `nextRemoteRevocationNumber = 6`

So when `localCommit.index == nextRemoteRevocationNumber + 1` we must retransmit the revocation.

```
          local              remote
            |                   |
            |  (no pending sig) |
 commit = 6 |                   | next rev = 6
            |<----- sig 7 ------|
 commit = 7 |                   |
            |-- rev 6 --> ?     |
            |                   |
            |  (disconnection)  |
            |                   |
```

eclair-core/src/main/scala/fr/acinq/eclair/channel/Helpers.scala

@@ -322,10 +322,10 @@ object Helpers {
    */
   def checkLocalCommit(d: HasCommitments, nextRemoteRevocationNumber: Long): Boolean = {
     if (d.commitments.localCommit.index == nextRemoteRevocationNumber) {
-      // they just sent a new commit_sig, we have received it but they didn't receive our revocation
+      // we are in sync
       true
     } else if (d.commitments.localCommit.index == nextRemoteRevocationNumber + 1) {
-      // we are in sync
+      // they just sent a new commit_sig, we have received it but they didn't receive our revocation
       true
     } else if (d.commitments.localCommit.index > nextRemoteRevocationNumber + 1) {
       // remote is behind: we return true because things are fine on our side