Fix WarningsAsErrors property reference (#149)

Author: 0xfeeddeadbeef

src/build/Workleap.DotNet.CodingStandards.props

@@ -49,7 +49,7 @@
 
     <!-- Fails the build on CI or on release when a vulnerability is detected -->
     <WarningsAsErrors Condition="$(NuGetAuditTreatWarningsAsErrors) == 'true'">
-      (WarningsAsErrors);NU1900;NU1901;NU1902;NU1903;NU1904
+      $(WarningsAsErrors);NU1900;NU1901;NU1902;NU1903;NU1904
     </WarningsAsErrors>
   </PropertyGroup>