Update visualize_ovelay_two_imgs.md

[jmgordon1223]

Added .visualize to code block example to reflect the function being within the visualize sub-package

Describe your changes
Added .visualize to the example code block to reflect the function being within the visualize sub-package

Type of update
Is this a:

• Bug fix
• New feature or feature enhancement
• Update to documentation<----
• Work in progress

Associated issues
Reference associated issue numbers. Does this pull request close any issues?
#1016 ?
Additional context
Add any other context about the problem here.

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[Stylopidae1793]

Looks good!

[codecov]

Codecov Report

Merging #1016 (fdd39f1) into 4.x (2aaa0f5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              4.x    #1016   +/-   ##
=======================================
  Coverage   99.98%   99.98%           
=======================================
  Files         161      161           
  Lines        6833     6833           
=======================================
  Hits         6832     6832           
  Misses          1        1           

Flag	Coverage Δ
unittests	99.98% <ø> (ø)

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