Fix a typo in a comment

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/tests/scripts/all-core.sh b/tests/scripts/all-core.sh
index 5a26224..ccd7e59 100644
--- a/tests/scripts/all-core.sh
+++ b/tests/scripts/all-core.sh
@@ -172,7 +172,7 @@
         else
             CRYPTO_CONFIG_H='include/psa/crypto_config.h'
             # helper_armc6_build_test() relies on these being defined,
-            # but empty if the paths don't exits (as in 3.6).
+            # but empty if the paths don't exist (as in 3.6).
             PSA_CORE_PATH=''
             BUILTIN_SRC_PATH=''
         fi