[mlir][tosa] Update URL to TOSA specification in TOSA.md

- The existing URL is no longer valid, updated to the current one

Signed-off-by: Jerry Ge <jerry.ge@arm.com>
Change-Id: I41a837bde1f680c9bd731e362d3ddcbee537f36a

Author: Jerry-Ge

mlir/docs/Dialects/TOSA.md

@@ -5,7 +5,7 @@
 ## Rationale
 
 The MLIR TOSA dialect implements the [TOSA
-specification](https://developer.mlplatform.org/w/tosa/).  This document
+specification](https://www.mlplatform.org/tosa/tosa_spec.html). This document
 describes the decision process for how TOSA expresses operators in
 high level dialects.