docs: change io/ioutil to io/os packages

Change io/ioutil to io/os packages because io/ioutil package
is deprecated from 1.16:

TempDir => os.MkdirTemp

Details: https://go.dev/doc/go1.16#ioutil

Fixes: #3265

Signed-off-by: bin <bin@hyper.sh>

docs/Unit-Test-Advice.md

@@ -286,8 +286,8 @@ func TestSomething(t *testing.T) {
     assert := assert.New(t)
 
     // Create a temporary directory
-    tmpdir, err := ioutil.TempDir("", "")    
-    assert.NoError(err)             
+    tmpdir, err := os.MkdirTemp("", "")
+    assert.NoError(err)
 
     // Delete it at the end of the test
     defer os.RemoveAll(tmpdir)