docs: In api.md add the tools/ part before test-backend.
docs: In api.md add the tools/ part before test-backend.

In this section of the docs, two tools for testing openapi
documentation are mentioned. But for the second one, we
forgot to mention that the tool also resides in the tools/
folder (like the first one which explicitly mentions it).
This commit fixes that.

zulip/zulip,andersk/zulip,hackerkid/zulip,shubhamdhama/zulip,punchagan/zulip,timabbott/zulip,synicalsyntax/zulip,hackerkid/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,shubhamdhama/zulip,punchagan/zulip,showell/zulip,synicalsyntax/zulip,eeshangarg/zulip,synicalsyntax/zulip,hackerkid/zulip,zulip/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,andersk/zulip,kou/zulip,timabbott/zulip,punchagan/zulip,brainwane/zulip,kou/zulip,brainwane/zulip,zulip/zulip,rht/zulip,punchagan/zulip,rht/zulip,rht/zulip,showell/zulip,eeshangarg/zulip,shubhamdhama/zulip,brainwane/zulip,timabbott/zulip,timabbott/zulip,timabbott/zulip,showell/zulip,andersk/zulip,zulip/zulip,shubhamdhama/zulip,brainwane/zulip,showell/zulip,brainwane/zulip,synicalsyntax/zulip,kou/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,showell/zulip,synicalsyntax/zulip,eeshangarg/zulip,andersk/zulip,synicalsyntax/zulip,kou/zulip,showell/zulip,hackerkid/zulip,zulip/zulip,shubhamdhama/zulip,timabbott/zulip,brainwane/zulip,punchagan/zulip,showell/zulip,rht/zulip,punchagan/zulip,kou/zulip,shubhamdhama/zulip,brainwane/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,timabbott/zulip,eeshangarg/zulip,shubhamdhama/zulip,andersk/zulip