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

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>