LTOVisibility.rst: fix up syntax in example

Summary: Pretty self-evident. This example was missing an lparen. Added it, and fixed up the ASCII art.

Patch by Nick Black <dankamongmen@gmail.com>

Reviewers: pcc

Reviewed By: pcc

Subscribers: tejohnson, mehdi_amini, inglorion, hiraditya, steven_wu, dexonsmith, cfe-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D70765