message: Fix some C++ syntax errors in how-to docs.
  GitOrigin-RevId:
diff: @@ -297,20 +297,20 @@ to encrypt or decrypt data:
  ```cpp
  #include "tink/aead.h"
  #include "tink/keyset_handle.h"
  - #include "tink/aead/aead_factory.h"
  + #include "tink/aead_factory.h"
  // 1. Get a handle to the key material.
  KeysetHandle keyset_handle = ...;
  // 2. Get the primitive.
  - auto aead_result= AeadFactory.GetPrimitive(keyset_handle);
  + auto aead_result= AeadFactory::GetPrimitive(keyset_handle);
  if (!aead_result.ok()) return aead_result.status();
  auto aead = std::move(aead_result.ValueOrDie());
  // 3. Use the primitive.
  auto ciphertext_result = aead.Encrypt(plaintext, aad);
  - if (!ciphertext_result.ok()) return ciphertext.status();
  + if (!ciphertext_result.ok()) return ciphertext_result.status();
  auto ciphertext = std::move(ciphertext_result.ValueOrDie());
  ```
  @@ -322,19 +322,19 @@ symmetric key encryption](PRIMITIVES.md#hybrid-encryption):
  ```cpp
  #include "tink/hybrid_decrypt.h"
  #include "tink/keyset_handle.h"
  - #include "tink/hybrid/hybrid_decrypt_factory.h"
  + #include "tink/hybrid_decrypt_factory.h"
  // 1. Get a handle to the key material.
  KeysetHandle keyset_handle = ...;
  // 2. Get the primitive.
  - auto hybrid_decrypt_result= HybridDecryptFactory.GetPrimitive(keyset_handle);
  + auto hybrid_decrypt_result = HybridDecryptFactory::GetPrimitive(keyset_handle);
  if (!hybrid_decrypt_result.ok()) return hybrid_decrypt_result.status();
  auto hybrid_decrypt = std::move(hybrid_decrypt_result.ValueOrDie());
  // 3. Use the primitive.
  auto plaintext_result = hybrid_decrypt.Decrypt(ciphertext, context_info);
  - if (!plaintext_result.ok()) return plaintext.status();
  + if (!plaintext_result.ok()) return plaintext_result.status();
  auto plaintext = std::move(plaintext_result.ValueOrDie());
  ```
  