Cherry-pick: VertexAI fix RST title underline (#3292)#3293

aabmass merged 1 commit intoopen-telemetry:package-release/opentelemetry-instrumentation-vertexai/v2.0bxopen-telemetry/opentelemetry-python-contrib:package-release/opentelemetry-instrumentation-vertexai/v2.0bxfrom

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