minor #16631 [Form] Fix typo (gintautasp12)

This PR was submitted for the 5.4 branch but it was merged into the 4.4 branch instead.

Discussion
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[Form] Fix typo

Fixes a typo "Testings" -> "Testing"

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Commits
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c4d4ced Fix typo

Author: javiereguiluz

form/unit_testing.rst

@@ -134,7 +134,7 @@ variable exists and will be available in your form themes::
     the ``KernelTestCase`` instead and use the ``form.factory`` service to
     create the form.
 
-Testings Types Registered as Services
+Testing Types Registered as Services
 -------------------------------------
 
 Your form may be used as a service, as it depends on other services (e.g. the