Fix PR gdb/21606: SYMBOL_FUNCTIONS_DOMAIN misspelled in documentation

Fix PR gdb/21606: SYMBOL_FUNCTIONS_DOMAIN misspelled in documentation

Both Python and Guile documentations misspelled
SYMBOL_FUNCTIONS_DOMAIN, writing SYMBOL_FUNCTION_DOMAIN instead.  This
obvious commit fixes it.

gdb/doc/ChangeLog:
2017-06-20  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/21606
	* python.texi (Python representation of Symbols.): Replace
	SYMBOL_FUNCTION_DOMAIN by SYMBOL_FUNCTIONS_DOMAIN, fixing typo.
	* guile.texi (Guile representation of Symbols.): Likewise.