Fix .gitignore pattern

Summary:
`java/**.asc` is not a correct gitignore pattern
See https://git-scm.com/docs/gitignore for the list of allowed `**` patterns

It seems reasonable to assume that intention is `java/**/*.asc`

The reason why it bothers me is the fact that ripgrep parses .gitignore files
and complains about invalid pattern
https://github.com/BurntSushi/ripgrep
Closes https://github.com/facebook/rocksdb/pull/2214

Differential Revision: D5063030

Pulled By: yiwu-arbug

fbshipit-source-id: ddd6682b81f03134be15f20fd596130776b69695

.gitignore

@@ -59,7 +59,7 @@ rocksdb.h
 unity.cc
 java/crossbuild/.vagrant
 .vagrant/
-java/**.asc
+java/**/*.asc
 java/javadoc
 
 scan_build_report/