[gdb/python] Fix typos

Fix typos:
...
gdb/python/py-framefilter.c:749: indention ==> indentation
gdb/python/py-framefilter.c:837: indention ==> indentation
gdb/python/py-lazy-string.c:35: sting ==> string
gdb/python/py-progspace.c:119: Retun ==> Return
gdb/python/py-progspace.c:139: Retun ==> Return
...