Update error message for allowed characters in aggregation names (#41573)

Exception message thrown when specifying illegal characters did
no accurately described the allowed characters.  This updates the 
error message to reflect reality (any character except [, ] and >)

server/src/main/java/org/elasticsearch/search/aggregations/AggregatorFactories.java

@@ -75,7 +75,7 @@ public class AggregatorFactories {
             final String aggregationName = parser.currentName();
             if (!validAggMatcher.reset(aggregationName).matches()) {
                 throw new ParsingException(parser.getTokenLocation(), "Invalid aggregation name [" + aggregationName
-                        + "]. Aggregation names must be alpha-numeric and can only contain '_' and '-'");
+                        + "]. Aggregation names can contain any character except '[', ']', and '>'");
             }
 
             token = parser.nextToken();