pkcs7: Fix typo in comment

Signed-off-by: Nick Child <nick.child@ibm.com>
diff --git a/include/mbedtls/pkcs7.h b/include/mbedtls/pkcs7.h
index b2cdabc..82f282b 100644
--- a/include/mbedtls/pkcs7.h
+++ b/include/mbedtls/pkcs7.h
@@ -66,7 +66,7 @@
 #define MBEDTLS_ERR_PKCS7_INVALID_FORMAT                   -0x5300  /**< The format is invalid, e.g. different type expected. */
 #define MBEDTLS_ERR_PKCS7_FEATURE_UNAVAILABLE              -0x5380  /**< Unavailable feature, e.g. anything other than signed data. */
 #define MBEDTLS_ERR_PKCS7_INVALID_VERSION                  -0x5400  /**< The PKCS7 version element is invalid or cannot be parsed. */
-#define MBEDTLS_ERR_PKCS7_INVALID_CONTENT_INFO             -0x5480  /**< The PKCS7 content info invalid or cannot be parsed. */
+#define MBEDTLS_ERR_PKCS7_INVALID_CONTENT_INFO             -0x5480  /**< The PKCS7 content info is invalid or cannot be parsed. */
 #define MBEDTLS_ERR_PKCS7_INVALID_ALG                      -0x5500  /**< The algorithm tag or value is invalid or cannot be parsed. */
 #define MBEDTLS_ERR_PKCS7_INVALID_CERT                     -0x5580  /**< The certificate tag or value is invalid or cannot be parsed. */
 #define MBEDTLS_ERR_PKCS7_INVALID_SIGNATURE                -0x5600  /**< Error parsing the signature */