Fix typo (closes #322)

l3build.dtx

@@ -714,7 +714,7 @@
 % For example, for the core \LaTeXe{} tests the main test files are contained
 % in a directory |testfiles|. To test font loading for \XeTeX{} and \LuaTeX{}
 % there are a second set of tests in |testfiles-TU| which use the short
-% |build-TU.lua| file shown in Listing~\vref{lst:configs}. To run both sets of
+% |config-TU.lua| file shown in Listing~\vref{lst:configs}. To run both sets of
 % tests, the main |build.lua| file contains the setting
 % |checkconfigs = {"build", "config-TU"}|. This will cause \pkg{l3build} to run
 % first using no additional settings (\emph{i.e.}~reading the normal