Added missing customHtml into landing page fixture.

[shinde-rahul]

Q	A
Bug fix? (use the a.b branch)	[ ]
New feature/enhancement? (use the a.x branch)	[ ]
Deprecations?	[ ]
BC breaks? (use the c.x branch)	[ ]
Automated tests included?	[ ]
Related user documentation PR URL	mautic/mautic-documentation#...
Related developer documentation PR URL	mautic/developer-documentation#...
Issue(s) addressed	Fixes #11471

Description:

Steps to test this PR:

1. Open this PR on Gitpod or pull down for testing locally (see docs on testing PRs here)
2. Run bin/console doctrine:fixtures:load
3. Attempt to launch the builder for any of the created emails or page

[codecov]

Codecov Report

Merging #12480 (dfa6412) into 4.4 (8d43a7f) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                4.4   #12480      +/-   ##
============================================
- Coverage     50.29%   50.29%   -0.01%     
  Complexity    35448    35448              
============================================
  Files          2145     2145              
  Lines        106431   106431              
============================================
- Hits          53528    53526       -2     
- Misses        52903    52905       +2     

see 1 file with indirect coverage changes