Fixed javadoc for core/dbSupport/SqlScript.java

As seen in com.googlecode.flyway.core.resolver.sql.PlaceholderReplacer, line [62](https://github.com/flyway/flyway/blob/master/flyway-core/src/main/java/com/googlecode/flyway/core/resolver/sql/SqlMigrationExecutor.java#L62), the placeholders are already replaced when calling SqlScript's constructor.