crypto/cipher, math/big: fix example names

Fixes (legit) vet warnings.
Fix some verb tenses while we're here.

Updates #11041

Change-Id: I27e995f55b38f4cf584e97a67b8545e8247e83d6
Reviewed-on: https://go-review.googlesource.com/27122
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/crypto/cipher/example_test.go

@@ -14,7 +14,7 @@ import (
 	"os"
 )
 
-func ExampleNewGCMEncrypter() {
+func ExampleNewGCM_encrypt() {
 	// The key argument should be the AES key, either 16 or 32 bytes
 	// to select AES-128 or AES-256.
 	key := []byte("AES256Key-32Characters1234567890")
@@ -40,7 +40,7 @@ func ExampleNewGCMEncrypter() {
 	fmt.Printf("%x\n", ciphertext)
 }
 
-func ExampleNewGCMDecrypter() {
+func ExampleNewGCM_decrypt() {
 	// The key argument should be the AES key, either 16 or 32 bytes
 	// to select AES-128 or AES-256.
 	key := []byte("AES256Key-32Characters1234567890")

src/math/big/floatexample_test.go

@@ -11,7 +11,7 @@ import (
 )
 
 func ExampleFloat_Add() {
-	// Operating on numbers of different precision.
+	// Operate on numbers of different precision.
 	var x, y, z big.Float
 	x.SetInt64(1000)          // x is automatically set to 64bit precision
 	y.SetFloat64(2.718281828) // y is automatically set to 53bit precision
@@ -26,8 +26,8 @@ func ExampleFloat_Add() {
 	// z = 1002.718282 (0x.faadf854p+10, prec = 32, acc = Below)
 }
 
-func Example_Shift() {
-	// Implementing Float "shift" by modifying the (binary) exponents directly.
+func ExampleFloat_shift() {
+	// Implement Float "shift" by modifying the (binary) exponents directly.
 	for s := -5; s <= 5; s++ {
 		x := big.NewFloat(0.5)
 		x.SetMantExp(x, x.MantExp(nil)+s) // shift x by s