docs: use proper rst markup for preformatted blocks

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

mercurial/helptext/rust.txt

@@ -28,7 +28,8 b' 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 b' 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