fix: documentation issues (#92, #132, #173)

cryptography/README.md

@@ -349,7 +349,7 @@ Future<void> main() async {
     keyPair: aliceKeyPair,
     remotePublicKey: bobPublicKey,
   );
-  final sharedSecretBytes = await aliceKeyPair.extractBytes();
+  final sharedSecretBytes = await sharedSecret.extractBytes();
   print('Shared secret: $sharedSecretBytes');
 }
 ```

cryptography/lib/src/cryptography/algorithms.dart

@@ -1605,7 +1605,7 @@ abstract class Poly1305 extends MacAlgorithm {
 /// import 'package:cryptography/cryptography.dart';
 ///
 /// Future<void> main() async {
-///   final algorithm = RsaPss(Sha256());
+///   final algorithm = RsaPss.sha256();
 ///
 ///   // Generate a key pair
 ///   final keyPair = await algorithm.newKeyPair();

cryptography/lib/src/cryptography/cipher.dart

@@ -348,7 +348,7 @@ abstract class Cipher {
   ///       secretKey: secretKey,
   ///       nonce: nonce,
   ///     );
-  ///     await encryptedSink.addStream(encryptedStream);
+  ///     await sink.addStream(encryptedStream);
   ///   } finally {
   ///     sink.close();
   ///   }