security-definitions-structure-validation.md has incorrect description

https://github.com/Azure/azure-openapi-validator/blame/master/docs/security-definitions-structure-validation.md#L11 should have a period at the end.

Running `autorest --validation --azure-validator readme.md` with
```json
...
"securityDefinitions": {
"azure_auth": {
"type": "oauth2",
"authorizationUrl": "https://login.microsoftonline.com/common/oauth2/authorize",
"flow": "implicit",
"description": "Azure Active Directory OAuth2 Flow",
"scopes": {
"user_impersonation": "impersonate your user account"
}
}
}
...
```
gives
```
Error: '$.securityDefinitions.azure_auth.description' has incompatible values (---
Azure Active Directory OAuth2 Flow.
, ---
Azure Active Directory OAuth2 Flow
).
```
Changing
`"description": "Azure Active Directory OAuth2 Flow"`
to
`"description": "Azure Active Directory OAuth2 Flow."`
solves the issue