Update linalg.md and example_blas_gemv.f90

[zoziha]

Description

1. Linking relying on fpm.toml requires adding the link keyword within the [build] section.
2. An error in the example_blas_gemv has been corrected.
   

[perazz]

LGTM with the suggested edit, thanks @zoziha.

Now that BLAS/LAPACK support is enabled with our local implementation, we will better address how to link against external libraries.
For CMake builds it will be easy (FindBLAS, FindLAPACK), but for fpm, there is no support yet and I think the best way to do it is to search for said libraries via metapackage.

This PR is cosmetic and I will merge if @jvdp1 also has no further comments.

[jvdp1]

Thank you @zoziha for this PR. LGTM. It can be merged after @perazz 's suggestion has been considered