.gitignore: Fix ACTUAL.txt -> ACTUAL.TXT (#73)

text-ui-test/runtest.bat|sh scripts generate a file ACTUAL.TXT.
However, .gitignore uses ACTUAL.txt, which means the generated
file will not be ignored by Git on non-Windows OS.

Let's update .gitignore as ACTUAL.txt -> ACTUAL.TXT

.gitignore

@@ -13,5 +13,5 @@ src/main/resources/docs/
 *.iml
 bin/
 
-/text-ui-test/ACTUAL.txt
+/text-ui-test/ACTUAL.TXT
 text-ui-test/EXPECTED-UNIX.TXT