patch 9.1.1252: typos in code and docs related to 'diffopt' "inline:"

Problem:  Typos in code and docs related to 'diffopt' "inline:".
          (after v9.1.1243)
Solution: Fix typos and slightly improve the docs.
          (zeertzjq)

closes: #16997

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 6c87da9..1b5e5c0 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ