Fix SubstringBetween length calculation (#29)

Author: p1va

src/DotnetDocument/Extensions/StringExtensions.cs

@@ -27,7 +27,8 @@ public static string SubstringBetween(this string text, string start, string end
             var endIndex = text.IndexOf(end, StringComparison.Ordinal);
 
             if (text.Contains(start) && text.Contains(end) && startIndex < endIndex)
-                return text.Substring(startIndex + start.Length, endIndex - end.Length - startIndex);
+                return text.Substring(startIndex + start.Length,
+                    endIndex - startIndex - start.Length);
 
             return string.Empty;
         }

test/DotnetDocument.Tests/StringExtensionsTests.cs

@@ -27,5 +27,17 @@ public void ShouldOnlySubstringDoubleTriangleBrackets(string text, string expect
             substring.ShouldBe(expected);
         }
 
+        [Fact]
+        public void ShouldSubstringWithDifferentTokenLengths()
+        {
+            const string text = "This is <!--some comment--> in text.";
+
+            // Act
+            var substring = text.SubstringBetween("<!--", "-->");
+
+            // Assert
+            substring.ShouldBe("some comment");
+        }
+
     }
 }