content: Modify install tag checkout to avoid beta

Modify the install.md git tag checkout to avoid selecting "beta" tags
as well.  This avoids new folks installing a beta version of Mastodon,
only to skip all fixes in rc builds until stable release.

Alternative if avoiding Perl regexp:
git tag -l | grep -v 'rc[0-9]*$' | grep -v 'beta[0-9]*$' | sort -V | tail -n 1

content/en/admin/install.md

@@ -141,7 +141,7 @@ Use git to download the latest stable release of Mastodon:
 
 ```bash
 git clone https://github.com/mastodon/mastodon.git live && cd live
-git checkout $(git tag -l | grep -v 'rc[0-9]*$' | sort -V | tail -n 1)
+git checkout $(git tag -l | grep -v -P '(rc|beta)[0-9]*$' | sort -V | tail -n 1)
 ```
 
 #### Installing the last dependencies {#installing-the-last-dependencies}