[mlir][doc] Fix usage of PatternApplicator.

The constructor of PatternApplicator doesn't have a constructor that
accepts only a `RewritePatternSet` as currently used in the example
code in PatternRewriter.md. Instead, one has to turn it into a
`FrozenRewritePatternSet`.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D125236

mlir/docs/PatternRewriter.md

@@ -283,7 +283,7 @@ public:
 
 /// Apply the custom driver to `op`.
 void applyMyPatternDriver(Operation *op,
-                          const RewritePatternSet &patterns) {
+                          const FrozenRewritePatternSet &patterns) {
   // Initialize the custom PatternRewriter.
   MyPatternRewriter rewriter(op->getContext());