Correct documentation of S3 bucket

Documentation added in aaf034b on Nov 17th 2016 says the S3 bucket can
be specified with `s3.bucket`, but commit c8bcc4c (#285) on Dec 18th
2016 used `s3bucket`. Instead of fixing the code (#552) to match the
documentation this commit changes just the documentation so that
existing configurations are not broken. Also, the `s3` object is passed
as is to `AWS.S3()`, which does not know the option `bucket` (but
silently ignores it in my test).

http://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/S3.html#constructor-property

Following the old documentation leads to this exception:

    2017-09-23T09:42:38.079Z - error:  MissingRequiredParameter: Missing required key 'Bucket' in params
        at ParamValidator.fail (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/param_validator.js:50:37)
        at ParamValidator.validateStructure (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/param_validator.js:61:14)
        at ParamValidator.validateMember (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/param_validator.js:88:21)
        at ParamValidator.validate (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/param_validator.js:34:10)
        at Request.VALIDATE_PARAMETERS (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/event_listeners.js:125:42)
        at Request.callListeners (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/sequential_executor.js:105:20)
        at callNextListener (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/sequential_executor.js:95:12)
        at /srv/hackmd/hackmd/node_modules/aws-sdk/lib/event_listeners.js:85:9
        at finish (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/config.js:315:7)
        at /srv/hackmd/hackmd/node_modules/aws-sdk/lib/config.js:333:9
        at Credentials.get (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/credentials.js:126:7)
        at getAsyncCredentials (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/config.js:327:24)
        at Config.getCredentials (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/config.js:347:9)
        at Request.VALIDATE_CREDENTIALS (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/event_listeners.js:80:26)
        at Request.callListeners (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/sequential_executor.js:101:18)
        at Request.emit (/srv/hackmd/hackmd/node_modules/aws-sdk/lib/sequential_executor.js:77:10)

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