doc: prevent overflowing <code> tag in rendered HTML

Add an apparently missing back-tick to fix a multi-line <code> section
on https://git-scm.com/docs/git-log which seems to have been caused by
commit 18fb7ffc ("pretty: respect color settings [...]", 2017-07-13).

Signed-off-by: Katrin Leinweber <katrin.leinweber@uni-konstanz.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -181,7 +181,7 @@ endif::git-rev-list[]
   `color.diff`, `color.ui`, or `--color`, and respecting the `auto`
   settings of the former if we are going to a terminal). `%C(auto,...)`
   is accepted as a historical synonym for the default (e.g.,
-  `%C(auto,red)`). Specifying `%C(always,...) will show the colors
+  `%C(auto,red)`). Specifying `%C(always,...)` will show the colors
   even when color is not otherwise enabled (though consider
   just using `--color=always` to enable color for the whole output,
   including this format and anything else git might color).  `auto`