Update the text about gcc5 support for Armv8 CE

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/sha256.c b/library/sha256.c
index 605b2b0..ca3fa5d 100644
--- a/library/sha256.c
+++ b/library/sha256.c
@@ -63,8 +63,8 @@
 #        pragma clang attribute push (__attribute__((target("crypto"))), apply_to=function)
 #        define MBEDTLS_POP_TARGET_PRAGMA
 #      elif defined(__GNUC__)
-         /* FIXME: GCC-5 claims crypto extension, but some intrinsic are missed.
-          *        Known miss intrinsic can be workaround.
+         /* FIXME: GCC 5 claims to support Armv8 Crypto Extensions, but some
+          *        intrinsics are missing. Missing intrinsics could be worked around.
           */
 #        if __GNUC__ < 6
 #          error "A more recent GCC is required for MBEDTLS_SHA256_USE_A64_CRYPTO_*"