fix relative link to USGS logo

docs/pages/About-the-NSHMP.md

docs/pages/Building-&-Running.md

docs/pages/Calculation-Configuration.md

docs/pages/Functional-PSHA.md

docs/pages/Ground-Motion-Models.md

docs/pages/Hazard-Model.md

docs/pages/Logic-Trees-&-Uncertainty.md

docs/pages/Magnitude-Frequency-Distributions.md

docs/pages/Model-Editions.md

docs/pages/Model-Files.md

docs/pages/Model-Structure.md

docs/pages/Rupture-Scaling-Relations.md

docs/pages/Source-Types.md

docs/pages/USGS-Models.md