fix link to Agda docs (#529)

src/plfa/part1/Naturals.lagda.md

@@ -941,7 +941,7 @@ Such a pragma can only be invoked once, as invoking it twice would
 raise confusion as to whether `2` is a value of type `ℕ` or type
 `Data.Nat.ℕ`.  Similar confusions arise if other pragmas are invoked
 twice. For this reason, we will usually avoid pragmas in future chapters.
-Information on pragmas can be found in the (Agda documentation)[https://agda.readthedocs.io/en/v2.6.1/language/pragmas.html].
+Information on pragmas can be found in the [Agda documentation](https://agda.readthedocs.io/en/v2.6.1/language/pragmas.html).
 
 
 ## Unicode