Fix manipulating property sources example in Javadoc for Configurable…

…Environment

The "manipulating property sources" example in the Javadoc for
`ConfigurableEnvironment` states that `MutablePropertySources`
expect a `Map<String,String>`; whereas it expects a
`Map<String,Object>`.

Closes spring-projectsgh-29693

spring-core/src/main/java/org/springframework/core/env/ConfigurableEnvironment.java

@@ -37,7 +37,7 @@
  * <pre class="code">
  * ConfigurableEnvironment environment = new StandardEnvironment();
  * MutablePropertySources propertySources = environment.getPropertySources();
- * Map&lt;String, String&gt; myMap = new HashMap&lt;&gt;();
+ * Map&lt;String, Object&gt; myMap = new HashMap&lt;&gt;();
  * myMap.put("xyz", "myValue");
  * propertySources.addFirst(new MapPropertySource("MY_MAP", myMap));
  * </pre>