Update api-conventions.asciidoc (#22439)

The example

```
/<logstash-{now/d-2d}>,<logstash-{now/d-1d}>,<logstash-{now/d}>/_search
```

shows escaped URL where `,  => %2C`, so I assume it should be escaped and be present in the table

docs/reference/api-conventions.asciidoc

@@ -118,6 +118,7 @@ The special characters used for date rounding must be URI encoded as follows:
 `|`:: `%7C`
 `+`:: `%2B`
 `:`:: `%3A`
+`,`:: `%2C`
 ======================================================
 
 The following example shows different forms of date math index names and the final index names