mpd: style fixes

Closes Homebrew/legacy-homebrew#47062.

Signed-off-by: Baptiste Fontaine <batifon@yahoo.fr>

Formula/mpd.rb

@@ -93,7 +93,7 @@ def install
     ENV.j1 # Directories are created in parallel, so let's not do that
     system "make", "install"
 
-    (etc+"mpd").install "doc/mpdconf.example" => "mpd.conf"
+    (etc/"mpd").install "doc/mpdconf.example" => "mpd.conf"
   end
 
   plist_options :manual => "mpd"
@@ -128,10 +128,10 @@ def plist; <<-EOS.undent
     sleep 2
 
     begin
-      assert_match /OK MPD/, shell_output("curl localhost:6600")
+      assert_match "OK MPD", shell_output("curl localhost:6600")
     ensure
-      Process.kill("SIGINT", pid)
-      Process.wait(pid)
+      Process.kill "SIGINT", pid
+      Process.wait pid
     end
   end
 end