Update README for mafull macOS rename.

Author: JBYoshi

README.md

@@ -6,6 +6,6 @@ Quickly check out SpongeForge and SpongeVanilla together sharing a SpongeCommon
 2. If you already have a SpongeForge and/or SpongeVanilla checkout, move them into your clone. If done properly, you should
 be able to run `git status` and not see any changes.
 3. If you have both a SpongeForge and SpongeVanilla checkout, **remove the SpongeCommon directory from one of them**.
-4. Pull up a system terminal (OS X/macOS/Linux) or Git Bash (Windows). `cd` into the repository directory.
+4. Pull up a system terminal (macOS/Linux) or Git Bash (Windows). `cd` into the repository directory.
 5. Type `./init`.
 6. Import into your preferred IDE.