Update PR template

Author: aramprice

.github/pull_request_template.md

@@ -2,8 +2,6 @@ NOTE: this repository uses a "Merge Forward" strategy
 
 Changes should be made in the earliest applicable branch, and
 merged forward through subsequent branches.
-1. Create a PR into the oldest branch (`ubuntu-<short_name>`)
-2. After this PR has been merged create a `merge-to-<next_short_name>` branch
-3. Merge `ubuntu-<short_name>` into `merge-to-<next_short_name>`
-4. Create a PR to merge `merge-to-<next_short_name>` into `ubuntu-<next_short_name>`
-5. Repeat as needed for subsequent branches
+1. PR should be created against the oldest stemcell branch, ex: `ubuntu-<short_name-N>`
+2. After this PR has been merged create a PR to merge `ubuntu-<short_name-N>` into `ubuntu-<short_name-N+1>`
+3. Repeat as needed for subsequent stemcell line branches