fix: typo "charcters" in README.md (#241)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

README.md

@@ -156,7 +156,7 @@ The session will have the name "GitHubActions" and be tagged with the following
 | Branch | GITHUB_REF |
 | Commit | GITHUB_SHA |
 
-_Note: all tag values must conform to [the requirements](https://docs.aws.amazon.com/STS/latest/APIReference/API_Tag.html). Particularly, `GITHUB_WORKFLOW` will be truncated if it's too long. If `GITHUB_ACTOR` or `GITHUB_WORKFLOW` contain invalid charcters, the characters will be replaced with an '*'._
+_Note: all tag values must conform to [the requirements](https://docs.aws.amazon.com/STS/latest/APIReference/API_Tag.html). Particularly, `GITHUB_WORKFLOW` will be truncated if it's too long. If `GITHUB_ACTOR` or `GITHUB_WORKFLOW` contain invalid characters, the characters will be replaced with an '*'._
 
 The action will use session tagging by default during role assumption. You can skip this session tagging by providing `role-skip-session-tagging` as true in the action's inputs: