Fix description of ProcessStartInfo.Environment

The docs incorrectly stated that the value of this property was null. It is never null and cannot be changed.

Author: AArnott

xml/System.Diagnostics/ProcessStartInfo.xml

@@ -649,7 +649,7 @@ If you use this property to set command-line arguments, <xref:System.Diagnostics
       </ReturnValue>
       <Docs>
         <summary>Gets the environment variables that apply to this process and its child processes.</summary>
-        <value>A generic dictionary containing the environment variables that apply to this process and its child processes. The default is <see langword="null" />.</value>
+        <value>A generic dictionary containing the environment variables that apply to this process and its child processes.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[