Update wording in comments

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/pkcs7.c b/library/pkcs7.c
index d4059d7..cf05afd 100644
--- a/library/pkcs7.c
+++ b/library/pkcs7.c
@@ -42,7 +42,7 @@
 #endif
 
 /**
- * Initializes the pkcs7 structure.
+ * Initializes the mbedtls_pkcs7 structure.
  */
 void mbedtls_pkcs7_init(mbedtls_pkcs7 *pkcs7)
 {