Documentation/diff-config: work around AsciiDoc misfortune

The line that happens to begin with indent followed by "3. " was
interpreted as if it was an enumerated list; just wrap the lines
differently to work it around for now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-config.txt

@@ -57,8 +57,8 @@ diff.statGraphWidth::
 	to all commands generating --stat output except format-patch.
 
 diff.context::
-	Generate diffs with <n> lines of context instead of the default of
+	Generate diffs with <n> lines of context instead of the default
-	3. This value is overridden by the -U option.
+	of 3. This value is overridden by the -U option.
 
 diff.external::
 	If this config variable is set, diff generation is not