Update expected error message due to changes in file provider (#876)

Author: HaoK

test/Config.KeyPerFile.Test/KeyPerFileTests.cs

@@ -28,7 +28,7 @@ public void DoesNotThrowWhenOptionalAndDirectoryDoesntExist()
         public void ThrowsWhenNotOptionalAndDirectoryDoesntExist()
         {
             var e = Assert.Throws<ArgumentException>(() => new ConfigurationBuilder().AddKeyPerFile("nonexistent", false).Build());
-            Assert.Contains("The directory name", e.Message);
+            Assert.Contains("The path must be absolute.", e.Message);
         }
 
         [Fact]
@@ -305,4 +305,4 @@ public Stream CreateReadStream()
             return new MemoryStream(Encoding.UTF8.GetBytes(_contents));
         }
     }
-}
+}