Fixed LATEX/ R typo from Regression Models 01_03

LATEX:
\sum_{i=1^n} => \sum_{i=1}^n
\beta_1 => \hat \beta_1

R Code:
Fixed missing `/`
Merge pull request #28 from clifflu/typo_RM_01_03_patch01

Fixed LATEX/ R typo from Regression Models 01_03