Fix: Wording of error messages

[localheinz]

This PR

• modifies the wording of error messages to be more consistent with error messages generated by phpstan/phpstan

Follows #39.

[codecov]

Codecov Report

Merging #42 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #42   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity       53     53           
=======================================
  Files            11     11           
  Lines           182    182           
=======================================
  Hits            182    182

Impacted Files	Coverage Δ	Complexity Δ
...rc/Methods/NoParameterWithNullDefaultValueRule.php	100% <100%> (ø)	6 <0> (ø)	⬇️
...res/NoParameterWithNullableTypeDeclarationRule.php	100% <100%> (ø)	4 <0> (ø)	⬇️
.../Functions/NoNullableReturnTypeDeclarationRule.php	100% <100%> (ø)	4 <0> (ø)	⬇️
...c/Closures/NoNullableReturnTypeDeclarationRule.php	100% <100%> (ø)	3 <0> (ø)	⬇️
...rc/Methods/NoNullableReturnTypeDeclarationRule.php	100% <100%> (ø)	4 <0> (ø)	⬇️
.../Functions/NoParameterWithNullDefaultValueRule.php	100% <100%> (ø)	5 <0> (ø)	⬇️
...ods/NoParameterWithNullableTypeDeclarationRule.php	100% <100%> (ø)	5 <0> (ø)	⬇️
src/Classes/AbstractOrFinalRule.php	100% <100%> (ø)	7 <0> (ø)	⬇️
...c/Closures/NoParameterWithNullDefaultValueRule.php	100% <100%> (ø)	5 <0> (ø)	⬇️
...ons/NoParameterWithNullableTypeDeclarationRule.php	100% <100%> (ø)	4 <0> (ø)	⬇️
... and 1 more

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