fix a typo (#65)

protocol/invalid-block-progression.md

@@ -15,7 +15,7 @@ when handling invalid batches.
 
 The derivation pipeline is set up as a series of stages. It takes work to progress from one stage to the next.
 Sometimes this work involves many remote HTTP requests, other times it requires in memory manipulation of
-data structures. Its not ideal when there is an error in a later stage that causes a lot of additional work
+data structures. It's not ideal when there is an error in a later stage that causes a lot of additional work
 to be done in an earlier stage to recover. In practice, this looks like some sort of invalid batch was deserialized
 and errors during its processing.
 
@@ -57,4 +57,4 @@ the batch queue invariant.
 
 - This adds risk around unsafe head reorgs. To date, there has not been an unsafe head reorg. Cases where batches would be
 dropped and then there is another chance to progress the safe head via submitting another batch would be turned into
-unsafe reorgs. The software and ops have been hardened after running it in production for some time.
+unsafe reorgs. The software and ops have been hardened after running it in production for some time.