Update 9.0.md - Fix typo in Unicode escape sequence (#43029)

Fixed a typo in the Unicode escape sequence. A forward slash was used instead of a backwards one.

Author: colejohnson66

docs/core/compatibility/9.0.md

@@ -36,7 +36,7 @@ If you're migrating an app to .NET 9, the breaking changes listed here might aff
 | [Altered UnsafeAccessor support for non-open generics](core-libraries/9.0/unsafeaccessor-generics.md) | Behavioral change   | Preview 6          |
 | [API obsoletions with custom diagnostic IDs](core-libraries/9.0/obsolete-apis-with-custom-diagnostics.md) | Source incompatible | (Multiple) |
 | [BigInteger maximum length](core-libraries/9.0/biginteger-limit.md) | Behavioral change | Preview 6  |
-| [BinaryReader.GetString() returns "/uFFFD" on malformed sequences](core-libraries/9.0/binaryreader.md) | Behavioral change | Preview 7  |
+| [BinaryReader.GetString() returns "\uFFFD" on malformed sequences](core-libraries/9.0/binaryreader.md) | Behavioral change | Preview 7  |
 | [Creating type of array of System.Void not allowed](core-libraries/9.0/type-instance.md) | Behavioral change   | Preview 1          |
 | [Default `Equals()` and `GetHashCode()` throw for types marked with `InlineArrayAttribute`](core-libraries/9.0/inlinearrayattribute.md) | Behavioral change   | Preview 6          |
 | [FromKeyedServicesAttribute no longer injects non-keyed parameter](core-libraries/9.0/non-keyed-params.md) | Behavioral change   | RC 1               |