[MLIR][Docs] Fix a small typo in documentation.

Just fixes a tiny typo in a link between 2 pages.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D93616

mlir/docs/PatternRewriter.md

@@ -238,7 +238,7 @@ between, and within dialects using a concept of "legality". This framework
 allows for transforming illegal operations to those supported by a provided
 conversion target, via a set of pattern-based operation rewriting patterns. This
 framework also provides support for type conversions. More information on this
-driver can be found [here](DialectConversion.nd).
+driver can be found [here](DialectConversion.md).
 
 ### Greedy Pattern Rewrite Driver