Fix typo

README.md

@@ -59,7 +59,7 @@ To run `Goptical` on these data files, execute the `gopt` command line utility b
 
 * G. H. Spencer and M. V. R. K. Murty, "General Ray-Tracing Procedure," J. Opt. Soc. Am. 52, 672-678 (1962). This paper presents generalized ray tracing equations that cover not only rotationally symmetric surfaces (including aspherics) but also diffraction gratings. The paper allows for surfaces to have their own local axes. 
 
-* R. E. Hopkins and R. Hanau, "Fundamental Methods of Ray Tracing," in Military Standardization Handdbook: Optical Design, MIL-HDBK 141, U.S. Defense Supply Agency, Washington, DC, 1962. This is the fifth chapter in the document. It covers ray tracing equations for rotationally symmetric surfaces including aspheric surfaces. The equations are presented in a form suited for implementation in computer programs. The final equations in this document are very similar to Feder's equations. This document goes into details of how these equations are derived. Daniel Malacara: Handbook of Optical Design has a description of the ray tracing equations found in this document. 
+* R. E. Hopkins and R. Hanau, "Fundamental Methods of Ray Tracing," in Military Standardization Handbook: Optical Design, MIL-HDBK 141, U.S. Defense Supply Agency, Washington, DC, 1962. This is the fifth chapter in the document. It covers ray tracing equations for rotationally symmetric surfaces including aspheric surfaces. The equations are presented in a form suited for implementation in computer programs. The final equations in this document are very similar to Feder's equations. This document goes into details of how these equations are derived. Daniel Malacara: Handbook of Optical Design has a description of the ray tracing equations found in this document. 
 
 * Bram de Greve, "Reflections and Refractions in Ray Tracing," 2004. This paper appears to be the source for the refraction equations originally used by GNU Optical. My plan is to use Feder's equations instead.