doc: fix pr 1486 formatting (ldoc bug) with nbsp (0xa0)

ldoc formatting has problems with code block lines that start with
spaces.  use non-blocking-space (unicode 0xa0) as a work around.

Signed-off-by: Justin Charette <charetjc@gmail.com>

docs/17-porting-tips.md

@@ -97,7 +97,7 @@ work. Remove this section.
 The `quit` menu command must be wrapped in a function, otherwise an error
 occurs due to mismatched argument types from the v4.0 `awful.menu` library.
 
-     -- {{{ Menu
+     -- {{{ Menu
      -- Create a laucher widget and a main menu
      myawesomemenu = {
        { "manual", terminal .. " -e man awesome" },
@@ -109,7 +109,7 @@ occurs due to mismatched argument types from the v4.0 `awful.menu` library.
 
 The textclock is now part of the `wibox` library, rename it.
 
-     -- {{{ Wibar
+     -- {{{ Wibar
      -- Create a textclock widget
     -mytextclock = `awful.widget.textclock`()
     +mytextclock = `wibox.widget.textclock`()