docs: Correcting grammatical errors in the installation guide document.

[MrLinMH]

Description

There is a grammatical error in the provided config.yaml configuration in the "Updating Admin API key" section of the APISIX installation guide, missing a colon.

Fixes # (issue)

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[kayx23]

🤔 if I'm not mistaken I think the original grammar works as well. I've seen it elsewhere,

e.g. in the FAQ doc:

and decoupled mode:

I remember having the same question as well (long time ago) and tested this configuration format; it did work.