docs: improve definition of a Backup (#5316)

After some building, I realized that a backup is a snapshot of a
resource, not the whole m365 service. Initially, I assumed that 1 backup
takes the whole service per tenant
(https://discord.com/channels/1022200980487557130/1022200981376745474/1231385151376719892).
This doc update should help clear the confusion more.

Maybe there is a better way to word it, so any other suggestions are
welcome.

---

#### Does this PR need a docs update or release note?

- [x] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [ ] ⛔ No

#### Type of change

- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [x] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

---------

Co-authored-by: ashmrtn <[email protected]>

website/docs/setup/concepts.md

@@ -21,7 +21,8 @@ application to connect to your *M365 tenant* and transfer data during backup and
 ## Corso concepts {#corso-concepts}
 
 * **Repository** refers to the storage location where Corso securely and efficiently stores encrypted *backups* of your
-*M365 Services* data. See [Repositories](../repos) for more information.
+*M365 Service*'s data. See [Repositories](../repos) for more information.
 
-* **Backup** is a copy of your *M365 Services* data to be used for restores in case of deletion, loss, or corruption of the
-original data. Corso performs backups incrementally, and each backup only captures data that has changed between backup iterations.
+* **Backup** is a copy of a resource of your *M365 Service*'s data to be used for restores in case of deletion, loss,
+or corruption of the original data. Corso performs backups incrementally, and each backup only captures data that has
+changed between backup iterations.