Update labels-custom-metadata.md (#4235)
Update labels-custom-metadata.md (#4235)

Change H1 headings to H2 and H2 headings to H3. Shoud the headings be starting from H2?
shubheksha/docker.github.io,docker/docker.github.io,JimGalasyn/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,shin-/docker.github.io,anweiss/docker.github.io,anweiss/docker.github.io,johnstep/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,JimGalasyn/docker.github.io,docker/docker.github.io,johnstep/docker.github.io,docker/docker.github.io,shubheksha/docker.github.io,londoncalling/docker.github.io,shubheksha/docker.github.io,shin-/docker.github.io,JimGalasyn/docker.github.io,thaJeztah/docker.github.io,johnstep/docker.github.io,johnstep/docker.github.io,johnstep/docker.github.io,JimGalasyn/docker.github.io,thaJeztah/docker.github.io,anweiss/docker.github.io,shin-/docker.github.io,docker/docker.github.io,shubheksha/docker.github.io,shin-/docker.github.io,JimGalasyn/docker.github.io,shubheksha/docker.github.io,docker/docker.github.io,anweiss/docker.github.io,shin-/docker.github.io,anweiss/docker.github.io,londoncalling/docker.github.io