all: rename Example test functions to prevent vet errors

Building x/text was being prevented by vet errors caused by Example
test functions of the form ExampleXxx where Xxx was not an identifier
declared in the package. This is corrected by renaming:
  ExampleCollator_Strings to ExampleCollator_strings in text/collate
  ExampleDecodeWindows1252 to Example_decodeWindows1252 in text/encoding
  ExampleSelect to ExampleSelectf in text/feature/plural

Fixes golang/go#68967

Change-Id: Id584a518af766676ab44fc49c12160e36f66542a
Reviewed-on: https://go-review.googlesource.com/c/text/+/606736
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tim King <taking@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

collate/sort_test.go

@@ -12,7 +12,7 @@ import (
 	"golang.org/x/text/language"
 )
 
-func ExampleCollator_Strings() {
+func ExampleCollator_strings() {
 	c := collate.New(language.Und)
 	strings := []string{
 		"ad",

encoding/example_test.go

@@ -16,7 +16,7 @@ import (
 	"golang.org/x/text/transform"
 )
 
-func ExampleDecodeWindows1252() {
+func Example_decodeWindows1252() {
 	sr := strings.NewReader("Gar\xe7on !")
 	tr := charmap.Windows1252.NewDecoder().Reader(sr)
 	io.Copy(os.Stdout, tr)

feature/plural/example_test.go

@@ -10,7 +10,7 @@ import (
 	"golang.org/x/text/message"
 )
 
-func ExampleSelect() {
+func ExampleSelectf() {
 	// Manually set some translations. This is typically done programmatically.
 	message.Set(language.English, "%d files remaining",
 		plural.Selectf(1, "%d",