change default Lua code execution to Mod4+x

Signed-off-by: Julien Danjou <julien@danjou.info>

awesomerc.lua.in

@@ -246,7 +246,7 @@ globalkeys = awful.util.table.join(
                   awful.util.getdir("cache") .. "/history")
               end),
 
-    awful.key({ modkey }, "F4",
+    awful.key({ modkey }, "x",
               function ()
                   awful.prompt.run({ prompt = "Run Lua code: " },
                   mypromptbox[mouse.screen],

manpages/awesome.1.txt

@@ -80,7 +80,7 @@ Window manager control
     Quit *awesome*.
 *Mod4 \+ F1*::
     Run prompt.
-*Mod4 \+ F4*::
+*Mod4 \+ x*::
     Run Lua code prompt.
 *Mod4 \+ Return*::
     Spawn terminal emulator.