[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