Fix probabilistic bounded staleness reference.

input/4_replication.md

@@ -108,7 +108,7 @@ For example:
 - Client-centric consistency models attempt to provide more intelligible consistency guarantees while allowing for divergence.
 - CRDTs (convergent replicated datatypes) exploit knowledge regarding the commutativity (and associativity) of specific operations on specific datatypes.
 - Confluence analysis (as in the Bloom language) uses information regarding the monotonicity of computations to maximally exploit disorder.
-- PBS (probabilistically bound staleness) uses simulation and information collected from a real world system to characterize the expected behavior of partial quorum systems.
+- PBS (probabilistically bounded staleness) uses simulation and information collected from a real world system to characterize the expected behavior of partial quorum systems.
 
 I'll talk about all of these a bit  further on, first; let's look at the replication algorithms that maintain single-copy consistency.