Add escape in the SQL statement in the flink engine document

[hililiwei]

the default is used in the SQL statement in the Flink document, but the example does not add escape, the statement cannot be run directly.
such as：

CREATE TABLE hive_catalog.default.sample (
    id BIGINT COMMENT 'unique id',
    data STRING
);

An error will be reported when the statement is executed：

[ERROR] Could not execute SQL statement. Reason:
org.apache.flink.sql.parser.impl.ParseException: Encountered ". default" at line 1, column 27.

modify the SQL：

CREATE TABLE hive_catalog.`default`.sample (
    id BIGINT COMMENT 'unique id',
    data STRING
);

the above statements can be executed normally.

[hililiwei]

master:a3b72fcaa33ec3db2b18d9ee50a2cc3023b23971