doc: fix typos and grammar in BUILDING.md & onboarding.md
#5
| Job | Run time |
|---|---|
| 2h 34m 48s | |
| 2h 34m 48s |
BUILDING.md & onboarding.md
#5
| Job | Run time |
|---|---|
| 2h 34m 48s | |
| 2h 34m 48s |