Remove C from code block

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/docs/3.0-migration-guide.md b/docs/3.0-migration-guide.md
index 1b673f7..9b3a0e9 100644
--- a/docs/3.0-migration-guide.md
+++ b/docs/3.0-migration-guide.md
@@ -725,7 +725,7 @@
 ### SSL key export interface change
 
 This affects users of the SSL key export APIs:
-```C
+```
     mbedtls_ssl_conf_export_keys_cb()
     mbedtls_ssl_conf_export_keys_ext_cb()
 ```