[mlir][doc] fix document link in Builtin Dialect document

[Avimitin]

Summary

The inline link to RankedTensorType in UnrankedTensorType section
and the inline link to MemRefType in UnrankedMemrefType section
are all invalid. This patch update these link.

[Avimitin]

I don't have permission to commit, please help me commit this change, thank you.

[aartbik]

Oops, just noticed your request. Will submit....

[Avimitin]

Thanks :)

mlir/include/mlir/IR/BuiltinTypes.td

let description = [{
     memory-space ::= attribute-value
     ```
 
     A `memref` type with an unknown rank (e.g. `memref<*xf32>`). The purpose of
     unranked memrefs is to allow external library functions to receive memref
     arguments of any rank without versioning the functions based on the rank.
     Other uses of this type are disallowed or will have undefined behavior.
 
-    See [MemRefType](#builtin_memref-memreftype) for more information on
+    See [MemRefType](#memreftype) for more information on
     memref types.
 
     Examples:
 
     ```mlir
     memref<*f32>
 
     // An unranked memref with a memory space of 10.
def Builtin_UnrankedTensor : Builtin_Type<"UnrankedTensor", [
   let description = [{
     Syntax:
 
     ```
     tensor-type ::= `tensor` `<` `*` `x` type `>`
     ```
 
     An unranked tensor is a type of tensor in which the set of dimensions have
-    unknown rank. See [RankedTensorType](#builtin_rankedtensor-rankedtensortype)
+    unknown rank. See [RankedTensorType](#rankedtensortype)
     for more information on tensor types.
 
     Examples:
 
     ```mlir
     tensor<*xf32>
     ```
   }];