Update reference docs to use PropertySourcesPlaceholderConfigurer

[gorisanson]

PropertyPlaceholderConfigurer has been deprecated as of Spring 5.2, in favor of PropertySourcesPlaceholderConfigurer, according to the doc comments following:

spring-framework/spring-beans/src/main/java/org/springframework/beans/factory/config/PropertyPlaceholderConfigurer.java

Lines 52 to 54 in 305055d

	* @deprecated as of 5.2; use {@code org.springframework.context.support.PropertySourcesPlaceholderConfigurer}
	* instead which is more flexible through taking advantage of the {@link org.springframework.core.env.Environment}
	* and {@link org.springframework.core.env.PropertySource} mechanisms.

[snicoll]

Thanks again @gorisanson