docs: Fix case of Cypress readme file

Fix the case of the cypress readme to fix dead links in the docs.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/docs/index.md b/docs/index.md
index d8ec950..978be11 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -45,7 +45,7 @@
   - [RIOT](readme-riot.md)
   - [Mbed OS](readme-mbed.md)
   - [Espressif](readme-espressif.md)
-  - [Cypress/Infineon](../boot/cypress/readme.md)
+  - [Cypress/Infineon](../boot/cypress/README.md)
   - [Simulator](../sim/README.rst)
 - Testing
   - [Zephyr](testplan-zephyr.md) - Zephyr test plan