fixup: target net472 - missed nuspec group

Author: JoshKeegan

src/xRetry.v3/xRetry.v3.nuspec

@@ -18,7 +18,7 @@
         <dependency id="xunit.core" version="[2.4.0,3.0.0)" />
       </group>
 
-      <group targetFramework=".NETFramework4.5.2">
+      <group targetFramework=".NETFramework4.7.2">
         <dependency id="xunit.core" version="[2.4.0,3.0.0)" />
       </group>      
     </dependencies>