Fix OSEHRA Logo in README

The README for the Documentation directory still uses the "Agent"
OSEHRA logo instead of the "Alliance" logo.

Point to the new image file.

OSEHRA-Id:http://issues.osehra.org/browse/OAT-142
Change-Id: I2fcbed98dae61cdd26db20c5a872043f3d36875d
Merge "Fix OSEHRA Logo in README"