axe two stray words in service.md

[covener]

changes for #14526 had either a stray "for a" or missed "service" as in "for a service".
Since it's quite lengthy and "for a service" is unnecessary in service.md, I opted to removed it.

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[covener]

/assign @gochist

[sftim]

Good spot
/lgtm

[gochist]

/unassign
/assign @kbarnard10

[bradtopol]

good catch

[bradtopol]

/assign

[bradtopol]

/lgtm
/approve

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