Change from OSX to macOS in README (#5545)

oh-my-zsh.sh

@@ -53,7 +53,7 @@ done
 
 # Figure out the SHORT hostname
 if [[ "$OSTYPE" = darwin* ]]; then
-  # OS X's $HOST changes with dhcp, etc. Use ComputerName if possible.
+  # macOS's $HOST changes with dhcp, etc. Use ComputerName if possible.
   SHORT_HOST=$(scutil --get ComputerName 2>/dev/null) || SHORT_HOST=${HOST/.*/}
 else
   SHORT_HOST=${HOST/.*/}