Correct undercase in setup.md (#288)

Author: PhilipVinc

docs/src/installing.md

@@ -26,7 +26,7 @@ controlled with the optional environment variables:
 - `JULIA_MPI_PATH`: the top-level installation directory of MPI.
 - `JULIA_MPI_LIBRARY_PATH`: the directory containing the MPI library files.
 - `JULIA_MPI_INCLUDE_PATH`: the directory containing the MPI header files.
-- `JULIA_MPI_CFlAGS`: C flags passed to the constant generation build (default: `-lmpi`)
+- `JULIA_MPI_CFLAGS`: C flags passed to the constant generation build (default: `-lmpi`)
 - `JULIA_MPICC`: MPI C compiler (default: `mpicc`)
 - `JULIA_MPIEXEC`: MPI startup command (default: `mpiexec`)