fix the display of "added in ..." versions (#6441)

Currently on https://www.netlifycms.org/docs/beta-features/ the "added in ..." version numbers are displaying as mailto: links, presumably because they contain an @ symbol. I assume this is undesirable? 

I clicked on one expecting to be taken to a change log associated with that version, but instead my email client opened.  Surrounding them in backticks makes them display as code instead. Alternatively we can turn them into actual links pointing to a change log, but I looked and couldn't seem to find a log (nor a matching release branch) on the website or on github. 

There is also another potential solution (adding an empty link) as explained here:
 https://stackoverflow.com/questions/59697538/making-it-so-email-addresses-arent-turned-into-hyperlinks 

But I haven't tested that option because I think surrounding them with backticks is a less confusing solution.

Author: tyleryoungblood

website/content/docs/beta-features.md

@@ -9,7 +9,7 @@ We run new functionality in an open beta format from time to time. That means th
 
 ## Working with a Local Git Repository
 
-***added in [email protected] / [email protected]***
+***added in `[email protected]` / `[email protected]`***
 
 You can connect Decap CMS to a local Git repository, instead of working with a live repo.
 
@@ -55,7 +55,7 @@ local_backend:
 
 ## GitLab and BitBucket Editorial Workflow Support
 
-***added in [email protected] / [email protected]***
+***added in `[email protected]` / `[email protected]`***
 
 You can enable the Editorial Workflow with the following line in your Decap CMS `config.yml` file: