Update docs-hpc.md (#65)

Following issue #64
Changes
In line 50
```bash
sudo singularity exec -w  /tmp/Centos-7.img make install
```
replaced for
```bash
sudo singularity exec -w -B /home /tmp/Centos-7.img make install
```

Author: ArangoGutierrez

pages/docs/admin-docs/docs-hpc.md

@@ -47,7 +47,7 @@ $ singularity exec /tmp/Centos-7.img ./configure --prefix=/usr/local
 $ singularity exec /tmp/Centos-7.img make
 $
 $ # Install OpenMPI into the container (notice now running as root and container is writeable)
-$ sudo singularity exec -w /tmp/Centos-7.img make install
+$ sudo singularity exec -w -B /home /tmp/Centos-7.img make install
 $
 $ # Build the OpenMPI ring example and place the binary in this directory
 $ singularity exec /tmp/Centos-7.img mpicc examples/ring_c.c -o ring