Bump ReportGenerator from 5.1.14 to 5.1.15 (#1032)

Author: dependabot[bot]

src/Polly.Specs/Polly.Specs.csproj

@@ -17,7 +17,7 @@
     <PackageReference Include="GitHubActionsTestLogger" Version="2.0.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.4.1" />
     <PackageReference Include="Moq" Version="4.18.4" />
-    <PackageReference Include="ReportGenerator" Version="5.1.14" PrivateAssets="all" />
+    <PackageReference Include="ReportGenerator" Version="5.1.15" PrivateAssets="all" />
     <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" PrivateAssets="all" />
   </ItemGroup>