doc/read-tree: remove obsolete remark

Earlier versions of `git read-tree` required the `--prefix` option value
to end with a slash. This restriction was eventually lifted without a
corresponding amendment to the documentation.

Signed-off-by: Andreas G. Schacker <andreas.schacker@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-read-tree.txt

@@ -81,12 +81,11 @@ OPTIONS
 * when both sides add a path identically.  The resolution
   is to add that path.
 
---prefix=<prefix>/::
+--prefix=<prefix>::
 	Keep the current index contents, and read the contents
 	of the named tree-ish under the directory at `<prefix>`.
 	The command will refuse to overwrite entries that already
-	existed in the original index file. Note that the `<prefix>/`
-	value must end with a slash.
+	existed in the original index file.
 
 --exclude-per-directory=<gitignore>::
 	When running the command with `-u` and `-m` options, the