Change the default code listing language to Swift.

This improves the failure case of a code listing that accidentally omits
its language -- instead of getting no (or incorrect) syntax
highlighting, it gets highlighted as Swift.  Most of the time, that will
be right in this book.

Confirmed no changes in the rendered HTML.

Co-authored-by: Kyle Murray <kyle_murray@apple.com>

Author: amartini51

Sources/TSPL/TSPL.docc/Info.plist

@@ -9,7 +9,7 @@
 	<key>CFBundleIdentifier</key>
 	<string>org.swift.tspl</string>
 	<key>CDDefaultCodeListingLanguage</key>
-	<string>markdown</string>
+	<string>swift</string>
 	<key>CFBundleDevelopmentRegion</key>
 	<string>en</string>
 	<key>CFBundleIconFile</key>