Clarify workflow_dispatch input description

We are currently using 'Release tag' to describe the required input
to our `workflow_dispatch` trigger. This is inaccurate - this field
actually requires a 'Release id', which I discovered when testing
GCM Core `apt-get` deployments yesterday. Updating so that the
description doesn't confuse folks running the workflow for a release
that is not 'latest'.

.github/workflows/release-apt-get.yml

@@ -6,7 +6,7 @@ on:
   workflow_dispatch:
     inputs:
       release:
-        description: 'Release Tag'
+        description: 'Release Id'
         required: true
         default: 'latest'
 

README.md

@@ -116,7 +116,7 @@ Or you can run the `git update-microsoft-git` command, which will run those brew
 Hippo (21.04). Take the following steps to set up and install based on the
 version you are running:
 
-### Bionic
+### Ubuntu 18.04 (Bionic)
 
 ```shell
 curl -sSL https://packages.microsoft.com/keys/microsoft.asc | sudo apt-key add -