[DOC] Improve grammar in CONTRIBUTING.md
[DOC] Improve grammar in CONTRIBUTING.md


xf-/meshviewer-1,Freifunk-Troisdorf/meshviewer,FreifunkBremen/meshviewer-ffrgb,FreifunkBremen/meshviewer-ffrgb,Freifunk-Troisdorf/meshviewer,freifunkMUC/meshviewer,xf-/meshviewer-1,Freifunk-Rhein-Neckar/meshviewer,FreifunkMD/Meshviewer,ffrgb/meshviewer,FreifunkMD/Meshviewer,ffrgb/meshviewer,Freifunk-Rhein-Neckar/meshviewer,Freifunk-Rhein-Neckar/meshviewer,freifunkMUC/meshviewer