Update src/borrow_check/region_inference/constraint_propagation.md

Co-Authored-By: lqd <remy.rakic+github@gmail.com>

Author: 2 people

src/borrow_check/region_inference/constraint_propagation.md

@@ -207,7 +207,7 @@ successors. Since SCCs form a DAG, we don't have to be concerned about
 cycles, though we do need to keep a set around to track whether we
 have already processed a given SCC or not. For each successor `S2`, once
 we have computed `S2`'s value, we can union those elements into the
-value for S1. (Although we have to be careful in this process to
+value for `S1`. (Although we have to be careful in this process to
 properly handle [higher-ranked
 placeholders](./placeholders_and_universes.html). Note that the value
 for S1 already contains the liveness constraints, since they were