[PATCH stable] docs: use proper rst markup for preformatted blocks

[Mads Kiilerich]

# HG changeset patch
# User Mads Kiilerich <mads at kiilerich.com>
# Date 1651574481 -7200
#      Tue May 03 12:41:21 2022 +0200
# Branch stable
# Node ID 1d257c4c15683ee998edcc7dd6caf5a4cb52c820
# Parent  f2ef6a4f918f59f2ff01e5d4dc01c94ea35bd32f
docs: use proper rst markup for preformatted blocks

The multiple lines were re-flowed to a single line, both in man page and html.

diff --git a/mercurial/helptext/rust.txt b/mercurial/helptext/rust.txt
--- a/mercurial/helptext/rust.txt
+++ b/mercurial/helptext/rust.txt
@@ -28,7 +28,8 @@ in progress. For more experimental work 
 Checking for Rust
 =================
 
-You may already have the Rust extensions depending on how you install Mercurial.
+You may already have the Rust extensions depending on how you install
+Mercurial::
 
   $ hg debuginstall | grep -i rust
   checking Rust extensions (installed)
@@ -46,7 +47,7 @@ version to use.
 Using pip
 ---------
 
-Users of `pip` can install the Rust extensions with the following command:
+Users of `pip` can install the Rust extensions with the following command::
 
   $ pip install mercurial --global-option --rust --no-use-pep517