Update "phpunit.xml.dist"

Author: jmini

samples/server/petstore/php-slim/phpunit.xml.dist

@@ -19,8 +19,8 @@
     </testsuites>
     <filter>
         <whitelist processUncoveredFilesFromWhitelist="true">
-            <directory suffix=".php">./lib//Api</directory>
-            <directory suffix=".php">./lib//Model</directory>
+            <directory suffix=".php">./lib/Api</directory>
+            <directory suffix=".php">./lib/Model</directory>
         </whitelist>
     </filter>
 </phpunit>