Improve Javadoc for @Value regarding supported expressions

[pxm7459]

Excerpt from the Javadoc for @Value:

public @interface Value {

	/**
	 * The actual value expression: for example {@code #{systemProperties.myProp}}.
	 */
	String value();

}

The actual value expression should be ${systemProperties.myProp}, using $ instead of #.

[sbrannen]

That # denotes that it is a SpEL (Spring Expression Language) expression.

As such, it is valid syntax.

Your proposed change represents a property placeholder.

However, the current documentation is unclear about what type of "expression" is being demonstrated.

Thus, I am repurposing this issue to improve the documentation.