[mlir][docs] Update MLIR's PatternRewriter documentation (#116183)

This PR adds the missing `const override` to the `rewrite` and
`matchAndRewrite` declaration in the Pattern Rewriter documentation as
described here:

https://github.com/llvm/llvm-project/blob/5cfa8baef33636827e5aa8dd76888c724433b53e/mlir/include/mlir/IR/PatternMatch.h#L237-L265

Author: jle-quel

mlir/docs/PatternRewriter.md

@@ -73,15 +73,15 @@ public:
     // otherwise.
     // ...
   }
-  void rewrite(Operation *op, PatternRewriter &rewriter) {
+  void rewrite(Operation *op, PatternRewriter &rewriter) const override {
     // The `rewrite` method performs mutations on the IR rooted at `op` using
     // the provided rewriter. All mutations must go through the provided
     // rewriter.
   }
 
   /// In this section, the `match` and `rewrite` implementation is specified
   /// using a single hook.
-  LogicalResult matchAndRewrite(Operation *op, PatternRewriter &rewriter) {
+  LogicalResult matchAndRewrite(Operation *op, PatternRewriter &rewriter) const override {
     // The `matchAndRewrite` method performs both the matching and the mutation.
     // Note that the match must reach a successful point before IR mutation may
     // take place.