Update ReadStringBuilderWithFixedLinesTests.cs

Author: SimonCropp

src/Verify.Tests/ReadStringBuilderWithFixedLinesTests.cs

@@ -15,7 +15,7 @@ public async Task All()
             if(!File.Exists(file))
             {
                 using var reader = IoHelpers.OpenRead(file);
-                await IoHelpers.ReadStringBuilderWithFixedLines(reader);
+                await reader.ReadStringBuilderWithFixedLines();
             }
         }