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

[ergawy]

Summary

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

[ftynse]

Thanks for spotting this!

mlir/docs/PatternRewriter.md

 
 ### Dialect Conversion Driver
 
 This driver provides a framework in which to perform operation conversions
 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
 
 This driver performs a post order traversal over the provided operations and
 greedily applies the patterns that locally have the most benefit. The benefit of
 a pattern is decided solely by the benefit specified on the pattern, and the
 relative order of the pattern within the pattern list (when two patterns have
 the same local benefit). Patterns are iteratively applied to operations until a
 fixed point is reached, at which point the driver finishes. This driver may be
 used via the following: `applyPatternsAndFoldGreedily` and
 `applyOpPatternsAndFold`. The latter of which only applies patterns to the
 provided operation, and will not traverse the IR.
 
 Note: This driver is the one used by the [canonicalization](Canonicalization.md)
 [pass](Passes.md#-canonicalize-canonicalize-operations) in MLIR.