update help page for Mac OS - fix OS naming

pages/install/mac/en.text

-- @title = "Bitmask for macOS"
-- @nav_title = 'macOS'
+- @title = "Bitmask for Mac OS"
+- @nav_title = 'Mac OS'
 
 h2. Download latest version
 
 Bitmask requires Mountain Lion or newer.
 
-<%= render({:partial => 'common/download_button'}, {:link => 'https://dl.bitmask.net/client/osx/Bitmask-OSX-latest.dmg', :text => 'Download for macOS'}) %>
+<%= render({:partial => 'common/download_button'}, {:link => 'https://dl.bitmask.net/client/osx/Bitmask-OSX-latest.dmg', :text => 'Download for Mac OS'}) %>
 
 Optionally, you can [[authenticate the signature => signature-verification]] for the Bitmask .dmg file using [[this signature file => https://dl.bitmask.net/client/osx/Bitmask-OSX-latest.dmg.asc]].