Update README.md

Corrected README.md

>\> lein beanstalk deploy dev
Created ###-0.0.1-20130508122413.war
Uploaded ###-0.0.1-20130508122413.war to S3 Bucket
Created new app version 0.0.1-20130508122413
Creating 'dev' environment (this may take several minutes)
Status Code: 400, AWS Service: AWSElasticBeanstalk, AWS Request ID: ###, AWS Error Code: ValidationError, AWS Error Message: 1 validation error detected: **Value 'dev' at 'environmentName' failed to satisfy constraint: Member must have length greater than or equal to 4**