How to display pageblock when click on button in visualforce page

Paste below code-

<apex:page standardController="Lead" Extensions="DisplayingListofrecords_Controller">
    <apex:form id="theform">
      <apex:pageBlock title="For buttons">
           <apex:commandButton action="{!ShowLeads}" value="Show Leads" reRender="theform"/>
            <apex:actionStatus startText="Requesting............"></apex:actionStatus>
            <apex:pageBlock title="Displaying List of accounts" id="Showlistofleads" rendered="true">
                <apex:dataTable value="{!leadlist}" var="a" cellpadding="4" border="1">
                    <apex:column >
                          <apex:facet name="header">Name</apex:facet>
                          {!a.Name}
                    </apex:column>  
                    <apex:column >
                          <apex:facet name="header">Company</apex:facet>
                          {!a.Company}
                    </apex:column>  
                    <apex:column >
                          <apex:facet name="header">Status</apex:facet>
                          {!a.Status}
                    </apex:column>  
                    <apex:column >
                          <apex:facet name="header">Industry</apex:facet>
                          {!a.Industry}
                    </apex:column>  
                    <apex:column >
                          <apex:facet name="header">Email</apex:facet>
                          {!a.Email}
                    </apex:column>  
                    <apex:column >
                          <apex:facet name="header">LeadSource</apex:facet>
                          {!a.LeadSource}
                    </apex:column>  
                    <apex:column >
                          <apex:facet name="header">Lead Score</apex:facet>
                          {!a.Lead_Score__c}
                    </apex:column>          
                </apex:dataTable>
            </apex:pageBlock>
        </apex:pageBlock>
    </apex:form>
</apex:page>

public with sharing class DisplayingListofrecords_Controller {
    Public List<Lead> leadlist {get;set;}
    public DisplayingListofrecords_Controller(ApexPages.StandardController controller) {
    }
    public PageReference ShowLeads()
    {
         leadlist = [select Name,Company,Status,Industry,Email,LeadSource from lead];
         return null;
    }
}