fix README typo: 'upon to the'

Author: softwareengineerprogrammer

README.rst

@@ -11,7 +11,7 @@ GEOPHIRES is a free and open-source geothermal techno-economic simulator.
 GEOPHIRES combines reservoir, wellbore, surface plant, and economic models to estimate the capital and operation and maintenance costs,
 instantaneous and lifetime energy production, and overall levelized cost of energy of a geothermal plant.
 Various reservoir conditions (EGS, doublets, etc.) and end-use options (electricity, direct-use heat, cogeneration) can be modeled.
-Users are encouraged to build upon to the GEOPHIRES framework to implement their own correlations and models.
+Users are encouraged to build upon the GEOPHIRES framework to implement their own correlations and models.
 See the `Documentation`_ section below for more information.
 
 This repository is the canonical source for the modern GEOPHIRES simulator, which succeeds the legacy `GEOPHIRES v2.0 <https://github.com/NREL/GEOPHIRES-v2>`__.