Improve wording; use PKCS #7 not PKCS7

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/ChangeLog.d/pkcs7-parser.txt b/ChangeLog.d/pkcs7-parser.txt
index ac477b1..b60d187 100644
--- a/ChangeLog.d/pkcs7-parser.txt
+++ b/ChangeLog.d/pkcs7-parser.txt
@@ -5,11 +5,11 @@
      - Only the signed-data content type, version 1 is supported.
      - Only DER encoding is supported.
      - Only a single digest algorithm per message is supported.
-     - Only 0 or 1 certificates are supported per message, which must be in
-       X.509 format.
+     - Certificates must be in X.509 format. A message must have either 0
+       or 1 certificates.
      - There is no support for certificate revocation lists.
      - The authenticated and unauthenticated attribute fields of SignerInfo
        must be empty.
      Many thanks to Daniel Axtens, Nayna Jain, and Nick Child from IBM for
-     contributing this feature. Thanks to Demi-Marie Obenour for contributing
+     contributing this feature, and to Demi-Marie Obenour for contributing
      various improvements, tests and bug fixes.