added missing validator to retry count text field

core/src/main/resources/lib/hudson/project/config-retryCount.jelly

@@ -27,7 +27,7 @@ THE SOFTWARE.
   <f:optionalBlock name="hasCustomScmCheckoutRetryCount" title="${%Retry Count}" checked="${it.hasCustomScmCheckoutRetryCount()}"
       help="/help/project-config/scmCheckoutRetryCount.html">
     <f:entry title="${%SCM checkout retry count}">
-      <f:textbox name="scmCheckoutRetryCount" value="${it.scmCheckoutRetryCount}"/>
+      <f:textbox clazz="number" name="scmCheckoutRetryCount" value="${it.scmCheckoutRetryCount}"/>
     </f:entry>
   </f:optionalBlock>
 </j:jelly>
\ No newline at end of file