am: align placeholder for --whitespace option with apply

`git am` passes the value given to its `--whitespace` option through
to the underlying `git apply`, and the value is called <action> over
there.  Fix the documentation for the command that calls the value
<option> to say <action> instead.

Note that the option help given by `git am -h` already calls the
value <action>, so there is no need to make a matching change there.

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

Documentation/git-am.txt

@@ -12,7 +12,7 @@ SYNOPSIS
 'git am' [--signoff] [--keep] [--[no-]keep-cr] [--[no-]utf8] [--no-verify]
 	 [--[no-]3way] [--interactive] [--committer-date-is-author-date]
 	 [--ignore-date] [--ignore-space-change | --ignore-whitespace]
-	 [--whitespace=<option>] [-C<n>] [-p<n>] [--directory=<dir>]
+	 [--whitespace=<action>] [-C<n>] [-p<n>] [--directory=<dir>]
 	 [--exclude=<path>] [--include=<path>] [--reject] [-q | --quiet]
 	 [--[no-]scissors] [-S[<keyid>]] [--patch-format=<format>]
 	 [--quoted-cr=<action>]
@@ -116,7 +116,7 @@ include::rerere-options.txt[]
 
 --ignore-space-change::
 --ignore-whitespace::
---whitespace=<option>::
+--whitespace=<action>::
 -C<n>::
 -p<n>::
 --directory=<dir>::