Fixed declaration of FlexContainerType

[Nazozen]

type FlexContainerType = string // alias type

Removed "=" in this declaration.
And "alias type" comment delete.

If this "=" is present, the linebot definition hover message will not be displayed in VSC on Mac OS.

Example
test.go code.

func testProc () {
var x linebot.TemplateAction
x.TemplateAction ()
_ = linebot.Client {}
_ = linebot.TextMessage {}
}

At this time, even if the mouse cursor is hovered over TemplateAction, Client, TextMessage, etc., the hint message is not displayed.

type FlexContainerType = string // alias type

This "=" is grammatical, but the VSC Go plugin doesn't seem to interpret correctly.

[codecov-io]

Codecov Report

Merging #182 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #182   +/-   ##
=======================================
  Coverage   73.41%   73.41%           
=======================================
  Files          24       24           
  Lines        1903     1903           
=======================================
  Hits         1397     1397           
  Misses        423      423           
  Partials       83       83

Impacted Files	Coverage Δ
linebot/flex.go	43.37% <ø> (ø)	⬆️

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[oklahomer]

LGTM 👍
Note: Continued work of #180 to replace type alias with a type definition to follow other declarations in this project. Confirmed the changes follow what was reviewed on the previous p-r: #180 (review) .