blame: correct name of config option in docs

As can be seen in files "Documentation/blame-options.txt" and
"builtin/blame.c", the name of this configuration option is
"blame.markUnblamableLines".

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/blame.txt

@@ -27,7 +27,7 @@ blame.ignoreRevsFile::
 	file names will reset the list of ignored revisions.  This option will
 	be handled before the command line option `--ignore-revs-file`.
 
-blame.markUnblamables::
+blame.markUnblamableLines::
 	Mark lines that were changed by an ignored revision that we could not
 	attribute to another commit with a '*' in the output of
 	linkgit:git-blame[1].