Get sources before building portaudio

You need the sources before you can build portaudio.

readme/WindowsBuild.md

@@ -55,22 +55,25 @@ pacman -S mingw-w64-x86_64-libzip
 
 TODO: Lua 5.3 is missing for MSYS2?
 
-### sndfile / PortAudio
-Build/Install portaudio with
-```bash
-windows-setup/build-portaudio.sh
-```
 
-## Get sources and build
+
+## Get sources
 
 ```bash
 git clone https://github.com/xournalpp/xournalpp.git
 cd xournalpp/
-mkdir build
-cd build/
 ```
 
+## sndfile / PortAudio
+Build/Install portaudio with
 ```bash
+windows-setup/build-portaudio.sh
+```
+
+## Build
+```bash
+mkdir build
+cd build/
 cmake ..
 make
 ```