Fix instructions for Catalina

Seems like `Blank.saver` doesn't work in Catalina but `Blank.qtz` does.

Didn't like the `.saver` version since its required Xcode so I hope that the next version of macOS supports `.qtz` as well.

Closes #2 

Ps.
Will try to update the Homebrew [Cast](https://github.com/theseal/homebrew-blank-screensaver)

README.md

@@ -1,6 +1,6 @@
 # Blank
 A blank (black) screensaver for MacOS/OS X.
-Tested on 10.12.6 and 10.14 but will propably work on any version.
+Tested on 10.12.6, 10.14 and 10.15.3 but will propably work on any version.
 
 Inspired by https://superuser.com/a/328670 but even smaller in size.
 
@@ -17,13 +17,7 @@ Inspired by https://superuser.com/a/328670 but even smaller in size.
 
 ### Without Homebrew
 
-* Mojave (10.14):
-  * Install the `Blank` screen saver
-```
-open Blank.saver
-```
-
-* Before Mojave (< 10.14):
+* For **all** versions of macOS **besides** Mojave:
 
   * Copy "Blank.qtz" to "~/Library/Screen Savers":
 ```
@@ -33,3 +27,9 @@ cp Blank.qtz ~/Library/Screen\ Savers/
 ```
 open /System/Library/PreferencePanes/DesktopScreenEffectsPref.prefPane
 ```
+
+* Mojave (10.14):
+  * Install the `Blank` screen saver
+```
+open Blank.saver
+```