Update TransferCodingWithQualityHeaderValue.xml (dotnet#5356)

The sentence:

> Represents an Accept-Encoding header **value.with** optional quality factor.

Should have a space rather than dot between the words ""value with** and thus read:

> Represents an Accept-Encoding header **value with** optional quality factor.

Author: ericmutta

xml/System.Net.Http.Headers/TransferCodingWithQualityHeaderValue.xml

@@ -37,7 +37,7 @@
     </Interface>
   </Interfaces>
   <Docs>
-    <summary>Represents an Accept-Encoding header value.with optional quality factor.</summary>
+    <summary>Represents an Accept-Encoding header value with optional quality factor.</summary>
     <remarks>
       <format type="text/markdown"><![CDATA[