[fix] Ignore sha2.lua in doc generation (#6229)

Otherwise it'll choke on the comments in that file.

See <https://github.com/koreader/koreader/pull/6195#issuecomment-638801075>.

Accidentally introduced in <https://github.com/koreader/koreader-base/pull/1105>.

doc/config.ld

@@ -22,4 +22,5 @@ file = {
     '../frontend',
     '../base/ffi',
     '../platform/android/luajit-launcher/assets',
+    exclude = {'../base/ffi/sha2.lua'},
 }