Fix jmxreceiver resource_attributes example (open-telemetry#30646)

Updating the jmxreceiver resource_attributes example in the readme.
Faced an error when starting the collector telling us it should be a
map, not a string.

**Testing:**
Changed to a map and it worked.

receiver/jmxreceiver/README.md

@@ -48,7 +48,9 @@ receivers:
     username: my_jmx_username
     # determined by the environment variable value
     password: ${env:MY_JMX_PASSWORD}
-    resource_attributes: my.attr=my.value,my.other.attr=my.other.value
+    resource_attributes:
+      my.attr: my.value
+      my.other.attr: my.other.value
     log_level: info
     additional_jars:
       - /path/to/other.jar
@@ -186,7 +188,7 @@ Additional JARs to be included in the java command classpath. This is currently
 
 ### resource_attributes
 
-List of resource attributes that will be applied to any metrics emitted from the metrics gatherer.
+Map of resource attributes that will be applied to any metrics emitted from the metrics gatherer.
 
 Corresponds to the `otel.resource.attributes` property.