Use correct env var name in a doc comment for Assume Role session name (#3854)

## Motivation and Context
The correct environment variable name for Assume Role session name is
`AWS_ROLE_SESSION_NAME`. We do use the correct name throughout the
codebase except for a doc comment. This PR will fix it.

----

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choice._

Author: ysaito1001

aws/rust-runtime/aws-config/Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "aws-config"
-version = "1.5.7"
+version = "1.5.8"
 authors = [
     "AWS Rust SDK Team <[email protected]>",
     "Russell Cohen <[email protected]>",

aws/rust-runtime/aws-config/Cargo.toml

@@ -19,7 +19,7 @@
 //! WebIdentityTokenCredentialProvider will load the following environment variables:
 //! - `AWS_WEB_IDENTITY_TOKEN_FILE`: **required**, location to find the token file containing a JWT token
 //! - `AWS_ROLE_ARN`: **required**, role ARN to assume
-//! - `AWS_IAM_ROLE_SESSION_NAME`: **optional**: Session name to use when assuming the role
+//! - `AWS_ROLE_SESSION_NAME`: **optional**: Session name to use when assuming the role
 //!
 //! ## AWS Profile Configuration
 //! _Note: Configuration of the web identity token provider via a shared profile is only supported