Removed invalid format specifier.

Former-commit-id: 4cb6197383b783a42f1f792f186e5af43a104566
Former-commit-id: ebbe9a9164c3e9c73e999c55da1320e3c0befdef
Former-commit-id: d5f7e5afd0cdc19f0e1fb5fdeb57b23880620c57

src/ImageProcessorCore/Numerics/LongRational.cs

@@ -273,7 +273,7 @@ namespace ImageProcessorCore
             StringBuilder sb = new StringBuilder();
             sb.Append(this.Numerator.ToString(provider));
             sb.Append("/");
-            sb.Append(this.Denominator.ToString("R", provider));
+            sb.Append(this.Denominator.ToString(provider));
             return sb.ToString();
         }