fixed memorycache config example (dotnet#4024)

memorycache, namedcache : attribute in add element should be physicalMemoryLimitPercentage instead of physicalMemoryPercentage

Author: Styxxy

docs/framework/configure-apps/file-schema/runtime/namedcaches-element-cache-settings.md

@@ -86,7 +86,7 @@ Specifies a collection of configuration settings for the named <xref:System.Runt
       <namedCaches>  
           <add name="default"   
                cacheMemoryLimitMegabytes="0"   
-               physicalMemoryPercentage="0"  
+               physicalMemoryLimitPercentage="0"  
                pollingInterval="00:02:00" />  
       </namedCaches>  
     </memoryCache>