docs: arm64: Fix indentation and doc formatting

Sphinx generates the following warnings for the arm64 doc
pages:

Documentation/arm64/memory.rst:158: WARNING: Unexpected indentation.
Documentation/arm64/memory.rst:162: WARNING: Unexpected indentation.

These indentations warnings can be resolved by utilising code
hightlighting instead.
Signed-off-by: NAdam Zerella <adam.zerella@gmail.com>
Signed-off-by: NWill Deacon <will@kernel.org>

Documentation/arm64/memory.rst

@@ -154,11 +154,18 @@ return virtual addresses to userspace from a 48-bit range.
 
 Software can "opt-in" to receiving VAs from a 52-bit space by
 specifying an mmap hint parameter that is larger than 48-bit.
+
 For example:
-    maybe_high_address = mmap(~0UL, size, prot, flags,...);
+
+.. code-block:: c
+
+   maybe_high_address = mmap(~0UL, size, prot, flags,...);
 
 It is also possible to build a debug kernel that returns addresses
 from a 52-bit space by enabling the following kernel config options:
+
+.. code-block:: sh
+
    CONFIG_EXPERT=y && CONFIG_ARM64_FORCE_52BIT=y
 
 Note that this option is only intended for debugging applications