Tiny fix in automation.h - unused return value removed

[latonita]

What does this implement/fix?

add_action() method in the template has return value, but it neither returned nor used.
Just replaced it with void

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes esphome/issues#5385

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.09%. Comparing base (4d8b5ed) to head (b2afcd4).
Report is 644 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6760      +/-   ##
==========================================
+ Coverage   53.70%   54.09%   +0.38%     
==========================================
  Files          50       50              
  Lines        9408     9627     +219     
  Branches     1654     1701      +47     
==========================================
+ Hits         5053     5208     +155     
- Misses       4056     4092      +36     
- Partials      299      327      +28     

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