Ensure multiline instructions in (onboarding) docs have `\` for line break

[srikanthccv]

The env won't be set/enabled otherwise. For example, if this is four lines command and non of env var will take effect

OTEL_RESOURCE_ATTRIBUTES=service.name=SampleFastAPI OTEL_EXPORTER_OTLP_ENDPOINT="https://ingest.in.signoz.cloud:443"
OTEL_EXPORTER_OTLP_HEADERS="signoz-access-token=xxxxxxxxxxxxxxxxxx"
OTEL_EXPORTER_OTLP_PROTOCOL=grpc
opentelemetry-instrument uvicorn main:app

The correct one is either a single line or multi-line with \.

OTEL_RESOURCE_ATTRIBUTES=service.name=SampleFastAPI OTEL_EXPORTER_OTLP_ENDPOINT="https://ingest.in.signoz.cloud:443" OTEL_EXPORTER_OTLP_HEADERS="signoz-access-token=xxxxxxxxxxxxxxxxxx" OTEL_EXPORTER_OTLP_PROTOCOL=grpc opentelemetry-instrument uvicorn main:app

or

OTEL_RESOURCE_ATTRIBUTES=service.name=SampleFastAPI OTEL_EXPORTER_OTLP_ENDPOINT="https://ingest.in.signoz.cloud:443" \
OTEL_EXPORTER_OTLP_HEADERS="signoz-access-token=xxxxxxxxxxxxxxxxxx" \
OTEL_EXPORTER_OTLP_PROTOCOL=grpc \
opentelemetry-instrument uvicorn main:app

cc @ankit01-oss @Calm-Rock