Fix message string

MateuszMaz · MateuszMaz · commit cf43e1cb5c3a · 2018-10-08T18:41:50.000+02:00
diff --git a/platform/mbed_error.c b/platform/mbed_error.c
@@ -350,7 +350,7 @@ static void print_error_report(mbed_error_ctx *ctx, const char *error_msg)
             break;
 
         case MBED_ERROR_CODE_ASSERTION_FAILED:
-            mbed_error_printf("Assertion failed, ", ctx->error_value);
+            mbed_error_printf("Assertion failed: ", ctx->error_value);
             break;
 
         default:
