Documentation/config.txt: camel-case lineNumber for consistency

lineNumber has casing that is inconsistent with surrounding options,
like color.grep.matchContext, and color.grep.matchSelected. Re-case this
documentation in order to be consistent with the text around it, and to
ensure that new entries are consistent, too.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

Documentation/config.txt

@@ -1181,7 +1181,7 @@ color.grep.<slot>::
 	filename prefix (when not using `-h`)
 `function`;;
 	function name lines (when using `-p`)
-`linenumber`;;
+`lineNumber`;;
 	line number prefix (when using `-n`)
 `match`;;
 	matching text (same as setting `matchContext` and `matchSelected`)