Fix Markdown links.

README.md

@@ -1,7 +1,7 @@
 crater
 ======
 
-**crater** is an emulator for the [Sega Game Gear](game gear), written in C.
+**crater** is an emulator for the [Sega Game Gear][game gear], written in C.
 
 [game gear]: https://en.wikipedia.org/wiki/Sega_Game_Gear
 
@@ -11,8 +11,8 @@ Why?
 While the internet is full of emulators for retro game systems, writing one is
 nevertheless a fun learning project.
 
-Crater is named after [31 Crateris](crateris), a star that was – for a short
-time in 1974 – misidentified as [a moon of Mercury](moon). Mercury was Sega's
+Crater is named after [31 Crateris][crateris], a star that was – for a short
+time in 1974 – misidentified as [a moon of Mercury][moon]. Mercury was Sega's
 codename for the Game Gear during development.
 
 [crateris]: http://www.astrostudio.org/xhip.php?hip=58587