Documentation: dma-buf: heaps: Fix code markup

Code snippets should be wrapped in double backticks to follow
reStructuredText semantics; the use of single backticks uses the
:title-reference: role by default, which isn't quite what we want.
Add double backticks to code snippets to fix this.

Reviewed-by: Maxime Ripard <[email protected]>
Signed-off-by: Jared Kangas <[email protected]>
Signed-off-by: Sumit Semwal <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: rh-jkangas

Documentation/userspace-api/dma-buf-heaps.rst

@@ -19,7 +19,7 @@ following heaps:
  - The ``cma`` heap allocates physically contiguous, cacheable,
    buffers. Only present if a CMA region is present. Such a region is
    usually created either through the kernel commandline through the
-   `cma` parameter, a memory region Device-Tree node with the
-   `linux,cma-default` property set, or through the `CMA_SIZE_MBYTES` or
-   `CMA_SIZE_PERCENTAGE` Kconfig options. Depending on the platform, it
+   ``cma`` parameter, a memory region Device-Tree node with the
+   ``linux,cma-default`` property set, or through the ``CMA_SIZE_MBYTES`` or
+   ``CMA_SIZE_PERCENTAGE`` Kconfig options. Depending on the platform, it
    might be called ``reserved``, ``linux,cma``, or ``default-pool``.