game entry file names changed to canonical version

games/zero-k.md

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+# Zero-K
+
+_A free multi-platform open source real-time strategy video game._
+
+- Home: http://zero-k.info/
+- Media: https://en.wikipedia.org/wiki/Zero-K
+- State: mature
+- Download: http://zero-k.info/Wiki/Download
+- Keywords: strategy, popular, real time
+- Code repository: https://github.com/ZeroK-RTS/Zero-K.git
+- Code language: Lua
+- Code license: GPL-2.0
+
+## Building
+