integration-docs: Update the example screenshot image paths.

of `jira-plugin` and `mercurial` integrations.

They were previously using the paths corresponding to the directory
names on python-zulip-api (`jira`, `hg`). Now, they're using their names
as used in the zulip/zulip repository.

Author: Niloth-p

zulip/integrations/hg/doc.md

@@ -83,4 +83,4 @@ Zulip notification.
 
 {!congrats.md!}
 
-![Mercurial bot message](/static/images/integrations/hg/001.png)
+![Mercurial bot message](/static/images/integrations/mercurial/001.png)

zulip/integrations/jira/doc.md

@@ -68,4 +68,4 @@ String issueBaseUrl = "https://jira.COMPANY.com/browse/"
 
 {!congrats.md!}
 
-![Jira bot message](/static/images/integrations/jira/001.png)
+![Jira bot message](/static/images/integrations/jira-plugin/001.png)