Unable to add as per User Guide Documentation

[nus-se-bot]

Describe the bug
When I use the add command to add a module, it does not add the module to the list. It's perhaps a User Guide documentation error.

To Reproduce
Steps to reproduce the behavior:

1. Open the app
2. Type in add c/CS2103T s/Y2S2 max/B_MINUS
3. It will not add the module to the list

Expected behavior
A module should be added to the list.

Screenshots

---

Reported by: @chung-ming
Severity: Medium

_{[original: nus-cs2103-AY1819S2/pe-dry-run#18]}

[alexchong711]

argument for setting max grade is maxg/ instead of max/
user guide will be updated