manual: correct the codeblock directive

[tchaikov]

it should start with three backticks not four of them. otherwise sphinx warns like:

/home/kefu/dev/cassandra-java-driver/docs/_source/manual/mapper/daos/getentity/index.md:111: WARNING: Lexing literal_block '@GetEntity\nProduct asProduct(Row row);\n\n@GetEntity\nProduct firstR
owAsProduct(ResultSet resultSet);\n```\n\n' as "java" resulted in an error at token: '`'. Retrying in relaxed mode.

in this change, let's use three backticks. and the warning disappears.

[tchaikov]

@tolbertam hi Andrew, thank you for your approval. but i don't have the write permission to this repo. and the CI failures do not look like related. is there anything else i can do to get this change merged?

[tchaikov]

@absurdfarce hi Bret, could you please take a look?

[bschoening]

@absurdfarce LGTM

[absurdfarce]

Apologies all.... too many balls in the air :(

Agreed on all counts... nice catch @tchaikov!