[mlir][bufferize][doc] Fix documentation of return type of `to_memref`.

This is my very first contact with this dialect, so I am not very
confident with this commit, but it seems like the op returns a memref,
not a tensor, so that's what comment about the result type should say.

[mlir][bufferization][doc] Improve typesetting of inline code. Fix Typo.

Reviewed By: pifon2a

Differential Revision: https://reviews.llvm.org/D130159