Make the output of the gears.object.signal example stable (#1703)

Previously it would print a table which would include the memory
location where the table was stored. Thus, this produces a different
result on every build.

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/examples/text/gears/object/signal.lua

@@ -2,6 +2,11 @@ local gears = require("gears") --DOC_HIDE
 
 local o = gears.object{}
 
+-- Add a __tostring metamethod for prettier output --DOC_HIDE
+getmetatable(o).__tostring = function() --DOC_HIDE
+    return "[obj]" --DOC_HIDE
+end --DOC_HIDE
+
 -- Function can be attached to signals
 local function slot(obj, a, b, c)
     print("In slot", obj, a, b, c)