Documentation: use brackets for optional arguments

The documentation for git blame used vertical bars for optional
arguments to -M and -C, which is unusual and potentially confusing.
Since most man pages use brackets for optional items, and that's
consistent with how we document the same options for git diff and
friends, use brackets here, too.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/blame-options.txt

@@ -76,7 +76,7 @@ include::line-range-format.txt[]
 	terminal. Can't use `--progress` together with `--porcelain`
 	or `--incremental`.
 
--M|<num>|::
+-M[<num>]::
 	Detect moved or copied lines within a file. When a commit
 	moves or copies a block of lines (e.g. the original file
 	has A and then B, and the commit changes it to B and then
@@ -92,7 +92,7 @@ alphanumeric characters that Git must detect as moving/copying
 within a file for it to associate those lines with the parent
 commit. The default value is 20.
 
--C|<num>|::
+-C[<num>]::
 	In addition to `-M`, detect lines moved or copied from other
 	files that were modified in the same commit.  This is
 	useful when you reorganize your program and move code