Fix up typos.

js/models/conversations.js

@@ -60,7 +60,7 @@
    *    - This is explicilty stored as a state in the conversation
    *
    * `isKeyExchangeCompleted` return whether we know for certain
-   *   that both of our preKeyBundles have been exhanged.
+   *   that both of our preKeyBundles have been exchanged.
    *    - This will be set when we receive a valid CIPHER or
    *      PREKEY_BUNDLE message from the other user.
    *        * Valid meaning we can decypher the message using the preKeys provided

libtextsecure/message_receiver.js

@@ -1048,7 +1048,7 @@ MessageReceiver.prototype.extend({
         // ref: libsignal-protocol.js:36120
         envelope.type === textsecure.protobuf.Envelope.Type.PREKEY_BUNDLE
       ) {
-      // We know for sure that keys are exhanged
+      // We know for sure that keys are exchanged
       if (conversation) {
         await conversation.setKeyExchangeCompleted(true);