[Spec] Suggestions from Liz Lynch
[Spec] Suggestions from Liz Lynch

Incorporates some suggestions from Liz Lynch:
* OpenAPI Specification (OAS) early and then OAS further on
* "a OAS" to "an OAS"
* Remove first three bullets from ToC

Signed-off-by: Rob Dolin <7737aa8e8feac0c2c2954de8a00c3ede85d230e8@microsoft.com>
OAI/OpenAPI-Specification,OAI/OpenAPI-Specification,OAI/OpenAPI-Specification