Fix typographical errors in .md files found by cspell

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/SECURITY.md b/SECURITY.md
index 26b77ab..33bbc2f 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -1,4 +1,4 @@
-## Reporting Vulneratibilities
+## Reporting Vulnerabilities
 
 If you think you have found an Mbed TLS security vulnerability, then please
 send an email to the security team at