Fix minor issues in user guide

*   Fix broken link to SCP download.

*   Remove requirement to install `ia32-libs`. This package is no
    longer available in current versions of Ubuntu and is no
    longer required when using the Linaro toolchain.

Change-Id: I9823d535a1d69136685754b7707b73e1eef0978d
Merge pull request #204 from danh-arm/dh/user-guide-fixes

Fix minor issues in user guide