docs: Fixed key generator initialize method call.

The guide within the KeyGenParameterSpec class now uses the correct
method (init() instead of initialize()) to initialize the key
generator in the examples that show how to create AES and HMAC keys.

Test: make ds-docs -j8

Bug: 69093664
Change-Id: I6a9cbe6decd895c2505538f6ad4be91cd9133714