Bump NUnit from 4.5.0 to 4.5.1 (#37)

Author: dependabot[bot]

MonthDiff.Tests/MonthDiff.Tests.csproj

@@ -10,7 +10,7 @@
 
     <ItemGroup>
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.3.0" />
-        <PackageReference Include="NUnit" Version="4.5.0" />
+        <PackageReference Include="NUnit" Version="4.5.1" />
         <PackageReference Include="NUnit3TestAdapter" Version="6.1.0" />
     </ItemGroup>