Outdated references to Mac OS X need to be updated to macOS in docs

[anxiousnix]

Bug

Description

In the documentation, there are numerous references to Mac OS X. Apple's desktop operating system, after a couple of changes, is now officially macOS. Furthermore, there are a couple references to macOS in the documentation, next to references to Mac OS X.

Thus, in order to look up to date (documentation functionally appears to work on the latest version of macOS "High Sierra"), and consistent, we need to convert all references to Apple's desktop operating system to macOS.

Steps to Reproduce

1. Read the SecureDrop documentation
2. Encounter references to Mac OS X, in a few cases near references to macOS
3. 🤔

User Stories

As a user, my confidence in the project is in part dependent on the quality of the documentation. Inconsistencies may give me pause as to the quality of documentation.