Update HttpRequestHeader.xml (#9814)

Description for IfModifiedSince header has a spelling mistake.
Sentence towards the end has - data and time instead of date and time.

Author: vinayk154

xml/System.Net/HttpRequestHeader.xml

@@ -1101,7 +1101,7 @@
       </ReturnValue>
       <MemberValue>30</MemberValue>
       <Docs>
-        <summary>The If-Modified-Since header, which specifies that the requested operation should be performed only if the requested resource has been modified since the indicated data and time.</summary>
+        <summary>The If-Modified-Since header, which specifies that the requested operation should be performed only if the requested resource has been modified since the indicated date and time.</summary>
       </Docs>
     </Member>
     <Member MemberName="IfNoneMatch">