Fix yarn installation command (#72)

Log1x · bartveneman · commit 5d3cdadbac29 · 2019-11-03T12:25:38.000+01:00
diff --git a/readme.md b/readme.md
@@ -16,7 +16,7 @@
 ```sh
 npm install -g wallace-cli
 # or
-yarn add --global wallace-cli
+yarn global add wallace-cli
 ```
 
 ## Usage
