Fix missing-word typo

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/SECURITY.md b/SECURITY.md
index e6d0bbf..d6c8f43 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -147,8 +147,8 @@
 
 **Warning!** Mbed TLS must not be used to sign untrusted CSRs unless extra
 validation is performed separately to ensure that they are compliant to the
-relevant specifications. This makes Mbed TLS on its own unsuitable use in a
-Certificate Authority (CA).
+relevant specifications. This makes Mbed TLS on its own unsuitable for use in
+a Certificate Authority (CA).
 
 However, Mbed TLS aims to protect against memory corruption and other
 undefined behavior when parsing certificates and CSRs. If a CSR or signed