Fix capitalisation on moduleHandler within TablefieldFormatter

Problem/Motivation

I'm subclassing TablefieldFormatter to customise the display of table cells. While running my changes through drupal-check, it claimed that it couldn't find the ModuleHandler property that gets used in the viewElements method. I can see the moduleHandler property being declared in the class, but case sensitivity seems to be the issue here.

Steps to reproduce

Run the module's codebase through drupal-check.

Proposed resolution

Change all references to $this->ModuleHandler to $this->moduleHandler to match the class property that gets declared.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.