[DOCS] Rename undeclared argument "length" to "maxCharacters"

The CropViewHelper does not have a "length" argument, but the
"maxCharacters" argument is mandatory. From the context it is clear that
the wrong argument was accidentally used here and that this needs to be
renamed.

Releases: main, 11.5, 10.4
Resolves: #97362
Change-Id: I4e8b443f48006024a5949a143f50ed309cdd3d9c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74293


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>

typo3/sysext/core/Documentation/Changelog/7.5/Feature-69459-ShowTt_contentPreviewInPageModuleViaFluid.rst

@@ -23,7 +23,7 @@ All content elements with CType "media" will then be rendered by the Fluid Templ
 .. code-block:: html
 
 	<h4>{header}</h4>
-	<f:format.crop length="200">{bodytext}</f:format.crop>
+	<f:format.crop maxCharacters="200">{bodytext}</f:format.crop>
 
 All properties of the tt_content record are available in the template directly.
 Any data of the flexform field `pi_flexform` is available with the property `pi_flexform_transformed` as an array.