docs: Update migration guide for primary key column rules (#209)

Author: jjurm

docs/guides/migration/v1-v2.md

@@ -32,7 +32,7 @@ nullability now becomes opt-in.
 ### Primary key columns may not be nullable
 
 While dataframely v1 merely emitted a warning, dataframely v2 now raises an exception if a primary key is designated
-as non-nullable. This aligns dataframely, for example, with SQL where primary key columns may not be nullable.
+as nullable. This aligns dataframely, for example, with SQL where primary key columns may not be nullable.
 
 ### Schema rules are now defined as classmethods