[llvm] fix docs WritingAnLLVMNewPMPass '%' in console can not be copied (PR #82759)

[via llvm-commits]

https://github.com/cyk2018 created https://github.com/llvm/llvm-project/pull/82759

Hello, I find that in our [page ](https://llvm.org/docs/WritingAnLLVMNewPMPass.html), we can not copy the % in our example .ll file. It seems the format is console, not llvm. I don't know if my fix method well. 
Thanks for your review.

>From ad5dd7056227fbf6381f01287763b04e1c533cab Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E9=99=88=E9=92=B0=E5=9D=A4?= <2959363314 at qq.com>
Date: Fri, 23 Feb 2024 20:45:56 +0800
Subject: [PATCH] fix docs WritingAnLLVMNewPMPass '%' in console can not be
 copied

---
 llvm/docs/WritingAnLLVMNewPMPass.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/docs/WritingAnLLVMNewPMPass.rst b/llvm/docs/WritingAnLLVMNewPMPass.rst
index ea30d637347f16..275f27bb64076f 100644
--- a/llvm/docs/WritingAnLLVMNewPMPass.rst
+++ b/llvm/docs/WritingAnLLVMNewPMPass.rst
@@ -164,7 +164,7 @@ it to run some LLVM IR through the pass.
 
   $ cat /tmp/a.ll
   define i32 @foo() {
-    %a = add i32 2, 3
+    \%a = add i32 2, 3
     ret i32 %a
   }