Correct one "Pure Data" to "Purr Data"

In this case we are talking about the app, not the language, so we
should use "Purr Data".

net.purrdata.PurrData.metainfo.xml

@@ -12,7 +12,7 @@
       user interface.
     </p>
     <p>
-        IMPORTANT: Running Pure Data under Flatpak is experimental.
+        IMPORTANT: Running Purr Data under Flatpak is experimental.
         The following are known limitations.
     </p>
     <ul>