update flyway.locations property doc to set correct default path

Tiny pull request to correct http://docs.spring.io/spring-boot/docs/current/reference/html/common-application-properties.html

```properties
flyway.locations=classpath:db/migrations # locations of migrations scripts
```

Whereas https://github.com/spring-projects/spring-boot/blob/master/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/flyway/FlywayProperties.java#L41 is defined to `db/migration` without `s`