bat: Use a real minus sign for the charging status

Previously, a simple hyphen (“-”) was used to show that the battery is
discharging. The minus sign is a distinct char, and also distinct from
the en-dash (“–”) and the em-dash (“—”).

With this commit, I put in a unicode “MINUS SIGN” (0x2212) which I
created with a Digraph in Vim `^k-2`.

Signed-off-by: Adrian C. (anrxc) <anrxc@sysphere.org>

widgets/bat.lua

@@ -30,7 +30,7 @@ local function worker(format, warg)
         ["Unknown\n"]     = "⌁",
         ["Charged\n"]     = "↯",
         ["Charging\n"]    = "+",
-        ["Discharging\n"] = "-"
+        ["Discharging\n"] = "−"
     }
 
     -- Check if the battery is present