Fix warnings in porting-guide.rst

Fix below warnings appeared in porting-guide.rst
WARNING: Title underline too short.

Change-Id: Ibc0eba0da72a53a5f9b61c49a8bf7a10b17bc3b8
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>

docs/getting_started/porting-guide.rst

@@ -1117,7 +1117,7 @@ the log output. The implementation should be robust to future changes that
 increase the number of log levels.
 
 Function : plat_get_soc_version()
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 ::
 
@@ -1132,7 +1132,7 @@ This function returns soc version which mainly consist of below fields
     soc_version[23:16] = JEP-106 identification code with parity bit for the SiP
 
 Function : plat_get_soc_revision()
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 ::