crypto: asymmetric_keys: fix some comments in pkcs7_parser.h

Drop the doubled word "the" in a comment.
Change "THis" to "This".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Ben Boeckel <mathstuf@gmail.com>
Cc: keyrings@vger.kernel.org