Rendering of `TABLESAMPLE` yields invalid `PERCENT` keyword for Postgres dialect about sqlglot

[georgesittas]

@whummer appreciate the reports, PRs would also be welcome for future issues you come across.

[georgesittas]

(1) requires the SYSTEM keyword to be present as default, and

FYI I don't think this is 100% correct. Postgres requires either BERNOULLI or SYSTEM to be used, but doesn't specify a default value, which is why there's a syntax error if the method is omitted. Snowflake on the other hand uses BERNOULLI as a default, so for the transpilation to be correct I made it so that we'll generate BERNOULLI for your example when targeting Postgres.

Let me know if I missed something and I'll amend my fix.