README: update the reference to the IERS 2010 conventions only because:
the solid.for program is based on the 2003 conventions, and updated against the latest leap seconds table.
there is no change in the used model/literature and the calculation accuracy of solid Earth tides between IERS 2003 and 2010 versions, as shown in Table 0.2 in Petit and Luzum (2010).
README: update the reference to the IERS 2010 conventions only because:
point: adjust annotation in the PSD plot