Add error checking in 05-awesomerc.md.lua (#1330)

io.open returns nil, an error message and an error code on failure. This
perfectly fits assert which will make the script fail with the error
message. Previously it would fail with "attempted to index a nil value".

Reference: https://www.reddit.com/r/awesomewm/comments/5kyqji/cant_build_git_awesome/
Signed-off-by: Uli Schlachter <psychon@znc.in>

docs/05-awesomerc.md.lua

@@ -1,6 +1,6 @@
 local filename, rcfile, new_rcfile = ...
 
-local f = io.open(filename, "w")
+local f = assert(io.open(filename, "w"))
 
 f:write[[# Default configuration file documentation
 
@@ -238,7 +238,7 @@ end
 
 -- Parse the default awesomerc.lua
 
-local rc = io.open(rcfile)
+local rc = assert(io.open(rcfile))
 
 local doc_block = false
 
@@ -293,6 +293,6 @@ f:write("\n")
 
 f:close()
 
-local rc_lua = io.open(new_rcfile, "w")
+local rc_lua = assert(io.open(new_rcfile, "w"))
 rc_lua:write(table.concat(output, "\n"))
 rc_lua:close()