a couple of minor XML doc fixes (#2702)

Author: jonsequitur

src/System.CommandLine/Completions/CompletionContext.cs

@@ -23,10 +23,14 @@ internal CompletionContext(ParseResult parseResult, string wordToComplete)
             WordToComplete = wordToComplete;
         }
 
+        /// <summary>
         /// The text of the word to be completed, if any.
+        /// </summary>
         public string WordToComplete { get; }
 
+        /// <summary>
         /// The parse result for which completions are being requested.
+        /// </summary>
         public ParseResult ParseResult { get; }
 
         /// <summary>

src/System.CommandLine/OptionValidation.cs

@@ -46,7 +46,7 @@ public static Option<FileSystemInfo> AcceptExistingOnly(this Option<FileSystemIn
         }
 
         /// <summary>
-        /// Configures an option to accept only values corresponding to a existing files or directories.
+        /// Configures an option to accept only values corresponding to existing files or directories.
         /// </summary>
         /// <param name="option">The option to configure.</param>
         /// <returns>The option being extended.</returns>