Correct the desc of google_composer_environment terraform resource

[mai-nakagawa]

In-place upgrade of Cloud Composer became only available using google-beta provider but not google provider by #5832. However, the related document was not aligned.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

composer: corrected the description of `image_version` field.

[modular-magician]

Oops! It looks like you're using an unknown release-note type in your changelog entries:

• REPLACEME

Please only use the types listed in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md.

[modular-magician]

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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 5 insertions(+), 4 deletions(-))
Terraform Beta: Diff ( 1 file changed, 5 insertions(+), 4 deletions(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 5 insertions(+), 4 deletions(-))
Terraform Beta: Diff ( 1 file changed, 5 insertions(+), 4 deletions(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[shuyama1]

LGTM! Thank you!