Update src/pk_encryption.rs

Co-authored-by: Damir Jelić <poljar@termina.org.uk>

src/pk_encryption.rs

@@ -181,7 +181,7 @@ impl PkDecryption {
     /// Create a [`PkDecryption`] object by unpickling a PkDecryption pickle in
     /// libolm legacy pickle format.
     ///
-    /// Such pickles are encrypted and need to first be decrypted using
+    /// Such pickles are encrypted and need to first be decrypted using a
     /// `pickle_key`.
     pub fn from_libolm_pickle(
         pickle: &str,