Documentation/git-update-ref.txt: drop “flag”

The other paragraphs on options say “With <option>,”.  Let’s be uniform.

Also add missing word “that”.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

Documentation/git-update-ref.txt

@@ -55,7 +55,7 @@ for reading but not for writing (so we'll never write through a
 ref symlink to some other tree, if you have copied a whole
 archive by creating a symlink tree).
 
-With `-d` flag, it deletes the named <ref> after verifying it
+With `-d`, it deletes the named <ref> after verifying that it
 still contains <old-oid>.
 
 With `--stdin`, update-ref reads instructions from standard input and