fix documentation typo about __decorator code generation

[charlypoly]

When __decorator applied to method, the last parameter (descriptor)
is null, not undefined

[msftclas]

Hi @wittydeveloper, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

TTYL, MSBOT;

[mhegazy]

Thanks for the change. this file is auto-generated by the build and not edited manually. the source file where this comment lives is https://github.com/Microsoft/TypeScript/blob/4701eb70e25682755eaed4ac13fdd31ecd850bd7/src/compiler/transformers/ts.ts#L1340-L1370. feel free to resubmit with the change done to the source file.