appdata: Fix no weird formatting validation error

https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines/#no-weird-formatting-1

Author: JoseExposito

data/app.appdata.xml.in.in

@@ -12,7 +12,7 @@
   <developer_name>José Expósito</developer_name>
   <launchable type="desktop-id">com.github.joseexposito.touche.desktop</launchable>
   <name>Touché</name>
-  <summary>Multi-touch Gestures</summary>
+  <summary>Multi-touch gestures</summary>
   <description>
     <p>Easily configure your touchpad and touchscreen multi-touch gestures with this GTK graphical user interface.</p>
     <p>For example, you can swipe up with 3 fingers to maximize a window or swipe left with 4 finger to switch to the next desktop.</p>