docs: fixup broken rst syntax

Seems I messed this up when converting things, and nobody noticed until
now!

Fixes: d4397c0048 ("docs: use envvar role for envvars")
Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18641>
(cherry picked from commit c816081eea)

.pick_status.json

@@ -202,7 +202,7 @@
         "description": "docs: fixup broken rst syntax",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "d4397c00485d8edb35d519e7036405527bb3e746"
     },

docs/envvars.rst

@@ -596,17 +596,17 @@ LLVMpipe driver environment variables
 VMware SVGA driver environment variables
 ----------------------------------------
 
-:envvar`SVGA_FORCE_SWTNL`
+:envvar:`SVGA_FORCE_SWTNL`
    force use of software vertex transformation
-:envvar`SVGA_NO_SWTNL`
+:envvar:`SVGA_NO_SWTNL`
    don't allow software vertex transformation fallbacks (will often
    result in incorrect rendering).
-:envvar`SVGA_DEBUG`
+:envvar:`SVGA_DEBUG`
    for dumping shaders, constant buffers, etc. See the code for details.
-:envvar`SVGA_EXTRA_LOGGING`
+:envvar:`SVGA_EXTRA_LOGGING`
    if set, enables extra logging to the ``vmware.log`` file, such as the
    OpenGL program's name and command line arguments.
-:envvar`SVGA_NO_LOGGING`
+:envvar:`SVGA_NO_LOGGING`
    if set, disables logging to the ``vmware.log`` file. This is useful
    when using Valgrind because it otherwise crashes when initializing
    the host log feature.