fix typo reported in #1199

doc/src/Build_basics.txt

@@ -137,9 +137,9 @@ simply loading the appropriate module before building LAMMPS.
 -D CMAKE_C_COMPILER=name              # name of C compiler
 -D CMAKE_Fortran_COMPILER=name        # name of Fortran compiler :pre
 
--D CMAKE_CXX_FlAGS=string             # flags to use with C++ compiler
--D CMAKE_C_FlAGS=string               # flags to use with C compiler
--D CMAKE_Fortran_FlAGS=string         # flags to use with Fortran compiler :pre
+-D CMAKE_CXX_FLAGS=string             # flags to use with C++ compiler
+-D CMAKE_C_FLAGS=string               # flags to use with C compiler
+-D CMAKE_Fortran_FLAGS=string         # flags to use with Fortran compiler :pre
 
 By default CMake will use a compiler it finds and it will add
 optimization flags appropriate to that compiler and any "accelerator