Fix typo (delegate) in documentation (#830)

Author: cdavid

src/Microsoft.AspNetCore.Http.Abstractions/Extensions/UseExtensions.cs

@@ -13,7 +13,7 @@ namespace Microsoft.AspNetCore.Builder
     public static class UseExtensions
     {
         /// <summary>
-        /// Adds a middleware delagate defined in-line to the application's request pipeline.
+        /// Adds a middleware delegate defined in-line to the application's request pipeline.
         /// </summary>
         /// <param name="app">The <see cref="IApplicationBuilder"/> instance.</param>
         /// <param name="middleware">A function that handles the request or calls the given next function.</param>
@@ -30,4 +30,4 @@ public static IApplicationBuilder Use(this IApplicationBuilder app, Func<HttpCon
             });
         }
     }
-}
+}