gitattributes.txt: fix typo

`UTF-16-LE-BOM` to `UTF-16LE-BOM`.

this closes https://github.com/git-for-windows/git/issues/2095

Signed-off-by: Yash Bhatambare <ybhatambare@gmail.com>
Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -344,7 +344,7 @@ automatic line ending conversion based on your platform.
 
 Use the following attributes if your '*.ps1' files are UTF-16 little
 endian encoded without BOM and you want Git to use Windows line endings
-in the working directory (use `UTF-16-LE-BOM` instead of `UTF-16LE` if
+in the working directory (use `UTF-16LE-BOM` instead of `UTF-16LE` if
 you want UTF-16 little endian with BOM).
 Please note, it is highly recommended to
 explicitly define the line endings with `eol` if the `working-tree-encoding`