Update api-conventions.asciidoc
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
@@ -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
