Fix aws sts docs and generator

[gordonbondon]

I've noticed that my previous PR for sts support had wrong code in docs and rails generator. Sorry about that. Here's a fix.

[PikachuEXE]

That's hard to spot!

[coveralls]

Pull Request Test Coverage Report for Build 440

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 66.019%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/generators/asset_sync/templates/asset_sync.rb	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
lib/asset_sync/config.rb	1	94.81%
lib/asset_sync/storage.rb	1	74.47%

Totals
Change from base Build 438:	0.2%
Covered Lines:	408
Relevant Lines:	618

---

💛 - Coveralls

[TravisBuddy]

Hey @gordonbondon,
Something went wrong with the build.

TravisCI finished with status errored, which means the build failed because of something unrelated to the tests, such as a problem with a dependency or the build process itself.

View build log

TravisBuddy Request Identifier: e405f9c0-b05f-11ea-ae15-c70b0cf57a1c

[PikachuEXE]

And released in 2.12.1