Fix 5 min guide with respect to @njit decl.
Fix 5 min guide with respect to @njit decl.

As title. `@njit` was not used in the introduction, but it's such
a commonly used alias is should be defined on the first page!

sklam/numba,stonebig/numba,gmarkall/numba,seibert/numba,IntelLabs/numba,numba/numba,numba/numba,stonebig/numba,seibert/numba,stuartarchibald/numba,jriehl/numba,IntelLabs/numba,stonebig/numba,sklam/numba,cpcloud/numba,numba/numba,seibert/numba,cpcloud/numba,numba/numba,gmarkall/numba,sklam/numba,jriehl/numba,gmarkall/numba,stonebig/numba,stuartarchibald/numba,IntelLabs/numba,gmarkall/numba,gmarkall/numba,jriehl/numba,IntelLabs/numba,stuartarchibald/numba,jriehl/numba,numba/numba,seibert/numba,sklam/numba,seibert/numba,stuartarchibald/numba,cpcloud/numba,IntelLabs/numba,sklam/numba,stuartarchibald/numba,stonebig/numba,jriehl/numba,cpcloud/numba,cpcloud/numba