doc: semihosting: fix section level

Wrong section level makes this section appear in the wrong table of
contents.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

doc/hardware/arch/semihost.rst

@@ -52,7 +52,7 @@ directory of the running process.
    semihost_close(fd);
 
 Additional Functionality
-########################
+************************
 
 Additional functionality is available by running semihosting instructions
 directly with :c:func:`semihost_exec` with one of the instructions defined