Fix unescaped description in requestBody

Created by: matthesrieke

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• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

This is a very small fix and re-uses existing functionality. It solves a glitch in OpenAPI 3 generation where the description of a requestBody of a POST operation was not escaped and caused code generators to have line breaks where no line breaks are allowed. Example:

    post:
      description: |
        Creates a new Job in the
        repository
      operationId: createJob
      requestBody:
        description: |
          Job to add to the 
          repository

was resulting in (lines 9-11):

    @ApiOperation(value = "", nickname = "createJob", notes = "Creates a new Job in the repository ", response = Job.class, tags={  })
    @ApiResponses(value = { 
        @ApiResponse(code = 201, message = "job response ", response = Job.class),
        @ApiResponse(code = 500, message = "unexpected error ", response = Error.class) })
    @RequestMapping(value = "/jobs",
        produces = { "application/json" }, 
        consumes = { "application/json" },
        method = RequestMethod.POST)
    default ResponseEntity<Job> createJob(@ApiParam(value = "Job to add to the
repository
",required=true)  @Valid @RequestBody Job job
) {

The fix is to re-use the existing escapeText method in DefaultCodegen.