Fix compilation failure on linux due to wrong case of package name

Correct package name in backwards-codecs from Lucene87 -> lucene87

It may cause no issues for case-insensitive filesystems such as on Mac
OS X or Windows, but it breaks on linux.
diff --git a/lucene/backward-codecs/src/test/org/apache/lucene/backward_codecs/Lucene87/Lucene87RWCodec.java b/lucene/backward-codecs/src/test/org/apache/lucene/backward_codecs/lucene87/Lucene87RWCodec.java
similarity index 100%
rename from lucene/backward-codecs/src/test/org/apache/lucene/backward_codecs/Lucene87/Lucene87RWCodec.java
rename to lucene/backward-codecs/src/test/org/apache/lucene/backward_codecs/lucene87/Lucene87RWCodec.java