i18n.txt: camel case and monospace "i18n.commitEncoding"

In 95791be750 (doc: camelCase the i18n config variables to improve
readability, 2017-07-17), the other i18n config variables were
camel cased. However, this one instance was missed.

Camel case and monospace "i18n.commitEncoding" so that it matches the
surrounding text.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
[jc: fixed 3 other mistakes that are exactly the same]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -79,7 +79,7 @@ OPTIONS
 	Pass `-u` flag to 'git mailinfo' (see linkgit:git-mailinfo[1]).
 	The proposed commit log message taken from the e-mail
 	is re-coded into UTF-8 encoding (configuration variable
-	`i18n.commitencoding` can be used to specify project's
+	`i18n.commitEncoding` can be used to specify project's
 	preferred encoding if it is not UTF-8).
 +
 This was optional in prior versions of git, but now it is the

Documentation/git-mailinfo.txt

@@ -53,7 +53,7 @@ character.
 	The commit log message, author name and author email are
 	taken from the e-mail, and after minimally decoding MIME
 	transfer encoding, re-coded in the charset specified by
-	i18n.commitencoding (defaulting to UTF-8) by transliterating
+	`i18n.commitEncoding` (defaulting to UTF-8) by transliterating
 	them.  This used to be optional but now it is the default.
 +
 Note that the patch is always used as-is without charset
@@ -61,7 +61,7 @@ conversion, even with this flag.
 
 --encoding=<encoding>::
 	Similar to -u.  But when re-coding, the charset specified here is
-	used instead of the one specified by i18n.commitencoding or UTF-8.
+	used instead of the one specified by `i18n.commitEncoding` or UTF-8.
 
 -n::
 	Disable all charset re-coding of the metadata.

Documentation/i18n.txt

@@ -38,7 +38,7 @@ mind.
   a warning if the commit log message given to it does not look
   like a valid UTF-8 string, unless you explicitly say your
   project uses a legacy encoding.  The way to say this is to
-  have i18n.commitencoding in `.git/config` file, like this:
+  have `i18n.commitEncoding` in `.git/config` file, like this:
 +
 ------------
 [i18n]