doc: stripspace: mention where the default comes from

Also quote `#` in line with the modern formatting convention.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-stripspace.adoc

@@ -37,7 +37,8 @@ OPTIONS
 -------
 -s::
 --strip-comments::
-	Skip and remove all lines starting with a comment character (default '#').
+	Skip and remove all lines starting with a comment character
+	(`core.commentChar`, default `#`).
 
 -c::
 --comment-lines::