macOS instead of OS X

### Summary

"In 2016, with the release of macOS 10.12 Sierra, the name was changed from OS X to macOS"
source :: https://en.wikipedia.org/wiki/MacOS#macOS

### Checklist

- [ ] If code changes were made then they have been tested.
    - [ ] I have updated the documentation to reflect the changes.
- [ ] This PR fixes an issue.
- [ ] This PR adds something new (e.g. new method or parameters).
- [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
- [X] This PR is **not** a code change (e.g. documentation, README, ...)

Author: Soheab

README.rst

@@ -30,7 +30,7 @@ To install the library without full voice support, you can just run the followin
 
 .. code:: sh
 
-    # Linux/OS X
+    # Linux/macOS
     python3 -m pip install -U discord.py
 
     # Windows
@@ -40,7 +40,7 @@ Otherwise to get voice support you should run the following command:
 
 .. code:: sh
 
-    # Linux/OS X
+    # Linux/macOS
     python3 -m pip install -U discord.py[voice]
 
     # Windows