Add missing 's' to `platforms`

The actual use of this option is `gem 'pg', platforms: :jruby`, not `gem 'pg', platform: :jruby`. This is a problem since gems that were not specific to the current Ruby platform were getting added to the gem list.

This was specially problematic because multiple DB adaptors were getting into the gem list when the same adaptor was being used for different platforms, which caused "More than one adapter gem found in Gemfile!" to be raised. Example:
```
gem 'activerecord-jdbcpostgresql-adapter', platforms: :jruby
gem 'pg', platforms: :mri
```