Update README.md

README.md

 This is OPAL >= 1.9.9. Be ware of the fact that the documentation is not
 anymore a seperate project. The documentation is in this repository in the 
 directory 'doc' (https://gitlab.psi.ch/OPAL/src/tree/master/doc). On machines 
-with LaTex the User's Reference Manual can be built in the same directory as the 
+with LaTeX the User's Reference Manual can be built in the same directory as the 
 code by configuring cmake using -DENABLE_DOCUMENTATION=1 to enable the compilation
 and -DEXCLUDE_DOC_FROM_ALL=1 to exclude it from compiling when make is called 
 without target. The User's Reference Manual can then be built by calling 'make doc'.
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