docs/gitweb.conf: config variable typo

The documentation for the feature 'snapshot' claimed
"This feature can be configured on a per-repository basis via
repository's `gitweb.blame` configuration variable"

Fixed to specify `gitweb.snapshot` as the variable name.

Signed-off-by: Frank Dana <ferdnyc@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitweb.conf.txt

@@ -684,7 +684,7 @@ compressed tar archive) and "zip"; please consult gitweb sources for
 a definitive list.  By default only "tgz" is offered.
 +
 This feature can be configured on a per-repository basis via
-repository's `gitweb.blame` configuration variable, which contains
+repository's `gitweb.snapshot` configuration variable, which contains
 a comma separated list of formats or "none" to disable snapshots.
 Unknown values are ignored.