Fix link to Rationale document from doc on tuple.

[ingomueller-net]

Summary

This also replaces the absolute link to the same document with a
relative one in the same file.

[rriddle]

Thanks!

[ingomueller-net]

I don't have permission, can someone land this for me?

mlir/include/mlir/IR/BuiltinTypes.td

let description = [{
     Syntax:
 
     ```
     // Target word-sized integer.
     index-type ::= `index`
     ```
 
     The index type is a signless integer whose size is equal to the natural
-    machine word of the target ( [rationale](https://mlir.llvm.org/docs/Rationale/Rationale/#integer-signedness-semantics) )
+    machine word of the target ( [rationale](../../Rationale/Rationale/#integer-signedness-semantics) )
     and is used by the affine constructs in MLIR.
 
     **Rationale:** integers of platform-specific bit widths are practical to
     express sizes, dimensionalities and subscripts.
   }];
   let extraClassDeclaration = [{
     static IndexType get(MLIRContext *context);
 
let description = [{
     tuple-type ::= `tuple` `<` (type ( `,` type)*)? `>`
     ```
 
     The value of `tuple` type represents a fixed-size collection of elements,
     where each element may be of a different type.
 
     **Rationale:** Though this type is first class in the type system, MLIR
     provides no standard operations for operating on `tuple` types
-    ([rationale](Rationale/Rationale/#tuple-types)).
+    ([rationale](../../Rationale/Rationale/#tuple-types)).
 
     Examples:
 
     ```mlir
     // Empty tuple.
     tuple<>
 
     // Single element