message: Fix missing word in rollups.md
  Also makes a small edit to split into two sentences.
diff: @@ -6,7 +6,7 @@ The Rust project has a policy that every pull request must be tested after merge
  before it can be pushed to master. As PR volume increases this can scale poorly,
  especially given the long (~3.5hr) current CI duration for Rust.
  -Enter rollups - changes that small, not performance sensitive, or not platform
  +Enter rollups! Changes that are small, not performance sensitive, or not platform
  dependent are marked with the `rollup` command to bors (`@bors r+ rollup` to
  approve a PR and mark as a rollup, `@bors rollup` to mark a previously approved
  PR, `@bors rollup-` to un-mark as a rollup). 'Performing a Rollup' then means
  