git-worktree.txt: fix indentation of example and text of 'add' command

When 4e85333197 (worktree: make add <path> <branch> dwim, 2017-11-26) added an example command in a literal code block, it neglected to insert a mandatory "+" line before the block. This omission resulted in both the literal code block and the (existing) paragraph following the block to be outdented, even though they should be indented under the 'add' sub-command along with the rest of the text pertaining to that command. Furthermore, the mandatory "+" line separating the code block from the following text got rendered as a leading character on the line ("+ If <commit-ish>...") rather than being treated as a formatting directive. Fix these problems by adding the missing "+" line before the example code block. Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>