tracing: Fix warnings when building htmldocs for function graph retval

When building htmldocs, the following warnings appear:

Documentation/trace/ftrace.rst:2797: WARNING: Literal block expected; none found.
Documentation/trace/ftrace.rst:2816: WARNING: Literal block expected; none found.

So fix it.

Link: https://lore.kernel.org/all/20230623143517.19ffc6c0@canb.auug.org.au/
Link: https://lkml.kernel.org/r/20230623071728.25688-1-pengdonglin@sangfor.com.cn

Fixes: 21c094d3 ("tracing: Add documentation for funcgraph-retval and funcgraph-retval-hex")
Signed-off-by: Donglin Peng <pengdonglin@sangfor.com.cn>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Documentation/trace/ftrace.rst

@@ -2792,7 +2792,7 @@ option, and these limitations will be eliminated in the future:
   especially when larger types are truncated, whether explicitly or implicitly.
   Here are some specific cases to illustrate this point:
 
-  **Case One**::
+  **Case One**:
 
   The function narrow_to_u8 is defined as follows::
 
@@ -2811,7 +2811,7 @@ option, and these limitations will be eliminated in the future:
   If you pass 0x123456789abcdef to this function and want to narrow it,
   it may be recorded as 0x123456789abcdef instead of 0xef.
 
-  **Case Two**::
+  **Case Two**:
 
   The function error_if_not_4g_aligned is defined as follows::