Fix stringifying of ~= in enums

[aidanhs]

Fixes #4069

The jira openapi spec includes "~=" as a value of an enum. This change ensures it gets stringified correctly.

You can see the problem by performing Rust codegen on https://developer.atlassian.com/cloud/jira/platform/swagger.v3.json:

$ java -jar openapi-generator-cli.jar generate -i $jiraspecpath -o jira-openapi -g rust --library reqwest --additional-properties packageName=jira-openapi
$ grep -A100 'pub enum Operator' jira-openapi/src/models/field_value_clause.rs
pub enum Operator {
    #[serde(rename = "=")]
    Equal,
    #[serde(rename = "!=")]
    Not_Equal,
    #[serde(rename = ">")]
    Greater_Than,
    #[serde(rename = "<")]
    Less_Than,
    #[serde(rename = ">=")]
    Greater_Than_Or_Equal_To,
    #[serde(rename = "<=")]
    Less_Than_Or_Equal_To,
    #[serde(rename = "in")]
    _In,
    #[serde(rename = "not in")]
    Not_in,
    #[serde(rename = "~")]
    Tilde,
    #[serde(rename = "~=")]
    ,
    #[serde(rename = "is")]
    Is,
    #[serde(rename = "is not")]
    Is_not,
}

The , on a line by itself is not valid Rust. With this change, that line becomes Tilde_Equal,

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[wing328]

LGTM