Correct styling of Mbed TLS in documentation

Several bits of documentation were incorrectly styling Mbed TLS
as MbedTLS.

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/tests/scripts/audit-validity-dates.py b/tests/scripts/audit-validity-dates.py
index 623fd23..5128dc7 100755
--- a/tests/scripts/audit-validity-dates.py
+++ b/tests/scripts/audit-validity-dates.py
@@ -276,7 +276,7 @@
 
     @staticmethod
     def find_test_dir():
-        """Get the relative path for the MbedTLS test directory."""
+        """Get the relative path for the Mbed TLS test directory."""
         return os.path.relpath(build_tree.guess_mbedtls_root() + '/tests')