Fix typo in HttpClientHandler.xml (#6606)

The sentence:

> Gets the collection of security certificates that are associated requests to the server.

Is missing the word "with" and should read:

> Gets the collection of security certificates that are associated **with** requests to the server.

xml/System.Net.Http/HttpClientHandler.xml

@@ -324,7 +324,7 @@ After NuGet package v4.3.2, the default value of <xref:System.Net.DecompressionM
         <ReturnType>System.Security.Cryptography.X509Certificates.X509CertificateCollection</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets the collection of security certificates that are associated requests to the server.</summary>
+        <summary>Gets the collection of security certificates that are associated with requests to the server.</summary>
         <value>The X509CertificateCollection that is presented to the server when performing certificate based client authentication.</value>
         <remarks>To be added.</remarks>
       </Docs>