.gitattributes: Use `demo/*`

According to https://github.com/github/linguist/issues/4644#issuecomment-530726508
2019-09-12 09:14:53 -04:00 · 2019-09-12 09:14:53 -04:00 · 46377a1fcb
--- a/.gitattributes
+++ b/.gitattributes
@ -2,7 +2,7 @@
 .gitignore text eol=lf

 # Override default for linguist-documentation.
-demo/ linguist-documentation=false
+demo/* linguist-documentation=false

 # Ethereum
 *.sol text linguist-language=Solidity eol=lf