docs: correctly style alert in `updating-content-github-ui` guide (#41250)
docs: correctly style alert in `updating-content-github-ui` guide (#41250)

This commit replaces the unknown `is-warning` CSS class from an alert in
the `updating-content-github-ui` guide with the `is-important` CSS class
(that causes the alert to be given an orange-ish background).

PR Close #41250

brandonroberts/angular,angular-indonesia/angular,ocombe/angular,ocombe/angular,brandonroberts/angular,mgechev/angular,gnomeontherun/angular,brandonroberts/angular,mgechev/angular,manekinekko/angular,angular/angular,JiaLiPassion/angular,shairez/angular,brandonroberts/angular,manekinekko/angular,gkalpak/angular,ocombe/angular,shairez/angular,brandonroberts/angular,brandonroberts/angular,JiaLiPassion/angular,shairez/angular,shairez/angular,gnomeontherun/angular,ocombe/angular,manekinekko/angular,manekinekko/angular,ocombe/angular,angular-indonesia/angular,gnomeontherun/angular,angular/angular,mgechev/angular,angular-indonesia/angular,shairez/angular,gkalpak/angular,manekinekko/angular,manekinekko/angular,mgechev/angular,JiaLiPassion/angular,angular-indonesia/angular,JiaLiPassion/angular,gkalpak/angular,angular/angular,gnomeontherun/angular,ocombe/angular,mgechev/angular,angular/angular,gkalpak/angular