all: fix main package in examples

The package with the main function should always have the name main.
This was not the case in three packages.

This was silently allowed before, but since a TinyGo change
(https://github.com/tinygo-org/tinygo/pull/1592) this now results in a
linker failure.

Perhaps this should result in a better error message in TinyGo. However,
the example code also needs to be fixed, so hence this PR.

examples/hcsr04/main.go

@@ -1,4 +1,4 @@
-package hcsr04
+package main
 
 import (
 	"machine"

examples/ssd1331/main.go

@@ -1,4 +1,4 @@
-package ssd1331
+package main
 
 import (
 	"machine"

examples/ssd1351/main.go

@@ -1,4 +1,4 @@
-package ssd1351
+package main
 
 import (
 	"machine"