Patch: correct LaTeX error in Linear Regression homework

The linear regression problem set uses an undefined macro, \sd.  This patch
removes that macro and just uses 'sd'.  It's not a fantastic workaround,
but the LaTeX code in that part of the document needs to be heavily
refactored anyway.
Merge branch 'patch_linearregression_problems' into dev

The linear regression problem set uses an undefined macro, \sd.  This patch
removes that macro and just uses 'sd'.  It's not a fantastic workaround,
but the LaTeX code in that part of the document needs to be heavily
refactored anyway.