message: Fix a bug in monkey patching locale guide
diff: @@ -746,13 +746,13 @@ Adding and replacing `format` tokens is easy with [`formatters`](#formatters) pr
  Deep clone the locale before monkey-patching to prevent mutability conflicts.
  ```javascript
  -import cloneDeep from 'lodash/cloneDeep'
  +import merge from 'lodash/merge'
  import format from 'date-fns/format'
  import originalLocale from 'date-fns/locale/en-US'
  const oneLetterWeekdays = ['S', 'M', 'T', 'W', 'T', 'F', 'S']
  -const locale = Object.assign(cloneDeep(originalLocale), {
  +const locale = merge(originalLocale, {
  formatters: {
  dd: function (date) {
  return oneLetterWeekdays[date.getUTCDay()]
  @@ -773,7 +773,7 @@ For the later, we will also need to add two new units: 'era' and 'eraYear'.
  In ECMAScript, year 0 is 1 B.C., year -1 is 2 B.C. etc.
  ```javascript
  -import cloneDeep from 'lodash/cloneDeep'
  +import merge from 'lodash/merge'
  import format from 'date-fns/format'
  import parse from 'date-fns/parse'
  import originalLocale from 'date-fns/locale/en-US'
  @@ -832,7 +832,7 @@ const parsers = {
  }
  }
  -const locale = Object.assign(cloneDeep(originalLocale), {
  +const locale = merge(originalLocale, {
  formatters,
  formattingTokensRegExp: buildTokensRegExp(formatters),
  units,
  