[mlir][memref] Remove extra brackets in doc

There is an (probably) unintended bracket in the documentation for `memref.subview`. See https://mlir.llvm.org/docs/Dialects/MemRef/#memrefsubview-mlirmemrefsubviewop below Example 5.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D132150